cosmiclady

This was from an earlier post, but I think will help out your situation. The most common problems leading to poor water quality are related to food debris and urine/stool in the water-leads to significant bacterial growth and nitrogen/ammonia accumulation...bad smell. This can also lead to shell/skin infections and other health problems.

I actually started making my own filters (due to cost), but I found it works well using a filter system that circulates the water in the tank 4 times each hour. Suck up the water from one end and have it re-enter the tank at the opposite end. I used a filter that was gravel filtration (removes debris) with activated charcoal (removes color) and nitrogen removing granules (Ammo-carb was a brand name). I have heard the Magnum canister filters work well. For turtles, you'll need a heavier filtration system because they are just messy-these will be external tank filters (a pump in the water with the filter outside the tank). I would clean mine out once each month. A number of the larger store bought filters will have a part called "biologic filtration". This is where beneficial bacteria grow in the filter-they help remove nitrogen waste and out compete bad bacteria to keep them at a minimum. The biologic filter takes 3 weeks or so to build up the good bacteria-normally on these plastic balls (Bio-balls) which you should not scrub clean when you change the filter.
I also tried to feed my turtles in a separate tank, but had no luck. I just got a siphon cleaner and a small net to suck up their waste... Some people have great luck with this and it does help keep the tank cleaner. Use a rubbermaid type container and dump the water after feeding.
You can make a balanced eco-system with plants, fish and snails, but you'll need a big tank or a small pond for this. More information on this is in the "basic care" file on this site (look under "files" in the left column)
Remember to get a good UVb light-esp important for younger turtles and change it every 9-12 months even if it doesn't burn out. Glass will not let any UVb light get through...
Hope that all made sense...filters get complicated but good water quality is really important for aquatic turtles. Make sure they can climb completely out of the water to bask and to let their shell dry.