To keep the water clear, you have to do 2 things, control suspended algae (green water), and filter out waste. It is also good to have some beneficial bacteria to help convert waste, and keep the water from getting toxic for fish and turtles.
Suspended Algae Control (green water)
These are the main factors contributing toward the growth of suspended Algae:
Warm water
High in nutrients (animal waste)
Sunlight
These are the things I have done to keep those under control (do as many of these as possible) UVB filter
This kills the algae, the bigger the better. I like the 32 watt ones for my larger ponds. The water flow through these filters has to be in a certain range, to fast and the UV won’t have time to kill the algae, to slow and you won’t pass enough water through the system. (there are normally recommended flow rates for each filter)
Shade
This keeps the water temp down, and cuts down on the sunlight energy that the algae needs. Shade can be, trees, floating water plants, or shade cloth. (Careful with floating plants, if your turtles eat them, they leave pieces which will clog up pumps; also trees tend to drop leaves which also clog things up)
Water Depth
The deeper the water, the less the sun will heat the water during the day, and the more constant the water temps will be in general (the ground will help keep the water temp constant). There will also be less sun energy exposed per gallon of water for the algae.
Water Quality
The more waste in the water the more food for the algae, so keeping this down, will help limit the population. Either a better filter or less animal load will help.
Mechanical filtration
Most filters on the market are for fish, and are not really made for filtering out the volume of waste that turtles create. I wanted to come up with a filter system with these properties:
Economical, easy to clean, keep water clear for turtles and fish.
The filter material I choose was something called Matala. This is something I had not heard of before, but looked promising. Sponges and foam material clogged up easily and was hard to clean out. Matala, pretty much rinses out, and has worked well so far. I got mine from here:
http://www.matalausa.com/subcat24.html
Construction
I started with a Rubbermaid roughneck 20 gallon trash can. Trashcans are good to use, because they are water tight, and will hold up to being left in the sun, many plastic products will get brittle if left out in the sun (as me how I know :). For the trash can all I had to do was drill 2 holes and make a slit (the size of your slit will vary depending on water flow, start small and enlarge it until you get the water height in the filter, and flow out of the filter balanced). The holes were 7/8” at the top for the water to come in (for a ¾” fitting, which is tight, but will work), 1 ¼” at the bottom for a drain (for cleaning) and a slit toward the top for the water to flow out like a water fall when filtering. I set mine up to be right over the pond, so just overflowing into the pond was fine for me. If you have to put it off to the side of the pond and need to direct the water back into the pond from the filter, you can also use 2, 1 ¼” holes at the top instead of the slit, and attach pvc to the holes (much like the drain) to direct the water back into the pond. If you do this, I recommend gluing these pvc joints (I don’t glue most pvc joints mentioned on this page, only where noted), if they fall off, it will drain your pond.
The water input is made up of ¾” pvc male and female threaded pieces. These are grey and from the electrical section of the hardware store. There is really not a need to put a washer on this connection, as it will be above the water line and not have to hold water. The drain is further explained here:
pvc.htm
I cut the Matala material into circles to fit the trashcan, this should be tight so that water does not just go around it, but don’t be too concerned if it is not perfect, but you do want a friction fit, so that the water pressure does not push up on the filter material when it starts catching solids. Keep in mind that the can is tapered, and that the piece on the bottom will be smaller than the piece on the top. Start with the most course you want to use on the bottom, and work your way up to the finer ones.
When stacking the filter media, I found that it is better to add some scrap pieces between the layers; it creates a place between them for waste to build up and settle. At first, I just put some scrap pieces between the layers, but then when cleaning, they fell out, and were a pain. Next, I tried zip tying the scraps to one of the layers, which held them in place, but then you can’t separate them to clean between them.
Then I tried putting the scraps through the posts, so they can move up and down for cleaning, but then they had a tendency to spin on the pvc when cleaning.
My last, and best, idea was to use scrap that is long enough to reach 2 posts diagonal from one another. This allows it to slide for cleaning, and does not spin around a single pole. (I alternated which poles I used on each layer)
Now I built a pipe to put the water coming in at the bottom of the filter, so that water flows up (this is easier than trying to pipe in the water directly to the bottom of the can). I started with a simple elbow and a hole in the center of all the filter layers and one pipe going to the bottom of the filter. This was ok, but hard to clean. Pulling out the layers was hard, and pulling the pipe through the hole all the time was wearing on the filter material making the hole bigger and not as good a fit. Then I came up with building a cage type thing to direct the water to the bottom of the filter and act as a material holder that can be pulled out as a unit. This has worked very well.
This is basically a ¾” pvc entering at the top, 90 degree elbow, a ¾” pvc T going to 2 more ¾” pvc T’s, which then reduce to ½”, then go to ½” pvc corner pieces (which are the only thing that the hardware store won’t have, I got them here http://www.littlegreenhouse.com/accessory/pvc.shtml) down ½” pvc to the bottom, where it is squared off with more ½” pvc. The pictures explain it better:
The only joints I have glued, are where the 4 ½ pieces go into the bottom. This is because I don’t want them pulling out when I tried to pick up on the top. You could also glue in the top joints, but mine have been pretty tight and not given me much trouble, and I can pull the top off if I want to, to clean or change out the filter pads on the rack (although I have not needed to). Not gluing the other joints allows me to twist an bend the joints to hook things up (mainly the top pieces to hook it up the filter input on the trashcan)
For the drain the page has more info on exactly what is used to make it:
pvc.htm.
This is a cheap easy way to put a drain at the bottom, to help get out the trash when cleaning it. Just turn the pipe down to start draining (just make sure to keep the pieces pushed together and not work them selves loose when twisting).
Pump Size
Another thing to keep in mind is pump size. There needs to be enough water flow to move the water in the pond through the filter, but you also can put to much pump on a filter (the bigger the filter, the bigger pump it can handle). I don’t have a rule of thumb, but I use 1200 GPH pumps on ponds that are made from 20’ X 20’ liners and are about 4’ deep. (you can also over pump your UV filter). Gravity is helping my system, as the water is slowly rising in the filter, and the solids in the water tend to want to settle. Pumping water to fast, would just pump the solids up to the top more.
Adding an anti-siphon valve
One issue with the way this was originally designed was that if the pump stopped (most commonly from a loss of power), then all the water from the filter would siphon back into the pond from the bottom of the filter, taking a lot of dirty water and garbage back into the pond. One way to solve this is a one way valve in line with the water flow, but getting a valve that would not restrict flow and will pass through the pond waist we are trying to filter out, would be expensive and large. An easier way is to add a smaller one way valve that will not allow water pressure out, but will allow air to be sucked in during a vacuum (when the pump runs normally there will be pressure in the system, when water is trying to siphon, it will be a vacuum). When air is allowed into the pipe the siphon is broken and the dirty water stays in the filter.
Adding any one way valve in the system would do, but I wanted it to be cheap, and last a long time, so I made one out of pvc parts and some scrap pond liner. I started with a ½” flat pvc cap (unfortunately most caps are rounded, which will not work as well, you should try to find a flat one ), and drilled a 3/16” hole in the center (exact size and placement is not important). Then took one of the ½” to ¾” bushings and traced a circle on the pond liner, and cut it out, cutting inside the lines. We want a circle that will fit into the cap without being tight. If it does not fall out of the cap when you flip it over, or lay flat just by dropping it in, it needs to be trimmed down more.
Then cut down one of the bushings. If we don’t cut it down, it will be to long and push the pond liner flap against the inside of the cap, and this will seal it up to much. We want a gap between the inside edge of the bushing, and the pond liner piece. Also we need to notch the bushing that we trimmed, so that the pond liner flap will not rest on that bushing and seal it (we want air to pass through between the pond liner and the cut down bushing).
Next is a small piece of ½ PVC pipe, length is not important, it just has to be long enough to be able to be pushed into the 2 bushings. Then we have another bushing (unmodified) and a ¾” pvc T fitting.
To assemble, drop the pond liner flap into the cap, the apply pvc glue to the shoulder end of the cut down bushing, we do NOT want any glue on the notched/trimmed end of the bushing, as that will gum up our valve. Next glue in the ½” pvc pipe to the bushing we just glued in, but be careful to only push it in a little bit. If you push in the pipe all the way (tightly like you normally would assemble pvc ) it will likely bind into the pond liner and not allow it to move around. At this point you can blow into the ½” pipe, and the flap should close and keep most of the air from flowing (a little getting out is ok, you can put a finger over the drilled hole to make sure no other air is escaping), then suck a little and you should get some air, if not, something is gumming up, or locking up the pond liner flap (which should be trapped in the end of the cap, but free to flop around in there). Next comes the unmodified bushing, and t fitting. If yours can be pushed together and not leak, no glue is required for these fittings, but I have found that most bushings don’t fit as well as other pvc fittings (which I don’t have to glue for pond pump systems) and I have to glue them.
Now replace the ¾” elbow fitting inside the filter with this one, and the siphon will be broken if the pump is stopped. (BTW it is ok if a little bit of water leaks from the one way valve, it is just going into the filter, and eventually will likely clog up with debris from the pond.) Below you can see the parts for each valve, a completely assembled valve, and, on the right, the valve installed on my filter.
Here you can see how slow the water moves up the filter, yet a decent volume of water flowing out the top. This is due to the width of the filter (trashcan). The wider, the filter, the slower the water will rise:
And lastly, here is a short video of how to clean it, basically:
Pull out water input
Start the drain
Pull out the filter material (easier if you let it drain first)
Rinse out material
Rinse out trashcan (can use the water input)
Raise up the drain pipe
Put material back in
Plug in the water input
Easier Cleaning
After cleaning these filters for a while I got couple of ideas to help with cleaning. First I got a small electric pressure washer. A hose works ok, but a pressure washer gets more stuff out and uses less water. Second I made a small wood rack to hold the filter media rack while washing. This keeps it off the ground and out of the dirty water you are washing off, and keeps it from rolling around when you are trying to wash it off (this is even worse with a pressure washer).
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