There are quite a few things you could/should be doing differently that will help with water quality. To type out everything with detailed explanations would take far too long, so here's a few reliable places to read that will help explain more in depth the following explanations: http://herp-info.webs.com/aquaticturtles... http://www.austinsturtlepage.com/Care/ca... 1) Your tank is too small/not enough water volume to dilute the massive amount of wast turtles produce. The general rule of thumb for aquatic turtle tank size is 10 gallons per inch of turtle.
2) You filter is too "small;" the filter should be rated for at least 2 - 3 times the size of the tank (assuming you are following the rule of thumb above.) You were correct to go with canister filtration. 3) Too much decor. Bare bottom tank (no gravel) is recommended for aquatic turtles.
Gravel holds poo and food particles which leads to poor water quality. Unless you use a gravel vacuum religiously, bare bottom is the way to go. 4) Keeping fish or any other living things with turtles just makes things much more complicated than it needs to be.
Just keep the turtle by itself. The fish and other living creatures add to the pollution/waste in the water which the turtle makes in abundance already. Not to mention most aquatic turtles, including RES, will eat almost anything, including fish and snails and such, especially when the turtles are bigger.
5) Use a waste degrader; it will help to break down the solid turtle waste into chemical form faster, where your filter can handle it better/easier. This will be something you add weekly for maintenance purposes. This will also help to keep the good bacterias in the tank healthier, as you would be basically adding a concentrated dose of good bacterias weekly (that's how the waste degrader works.) 6) Use aquarium/conditioning salt in the water; RES are tolerant to the aquarium salt and it will help a little with green algae (the brown in resistant), as it irritates it, and it also can help prevent disease.
You can choose to half dose it, or use as directed; more is NOT better. 7) Weekly partial water changed are recommended with turtles; you shouldn't remove more than 50% of the water at once (same rule of thumb for fish tanks.) Water changes help to dilute the dissolved wastes in the water thereby helping maintain better water quality. So, basically, I would recommend that you get a bigger tank and filter system for the turtle, and perhaps keep the current tank for the fish.
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