Red-eared sliders are really messy! Here's a couple of suggestions:
1. Small, frequent water changes. I would suggest 25% of the water twice a week to start with.
2. Dechlorinate the water. Turtles are not as sensitive as fish to any chlorine residuals that may be present in tap water, but the beneficial bacteria that break down waste are! They will be killed by the chlorine.
3. Consider a separate feeding container. Turtles will frequently poop ~ 20 minutes after eating, so if she can tolerate it, try feeding her in a separate container and just discard the water and waste so it never sees the terrarium.
4. Add beneficial bacteria (nitrifying bacteria) to any new filter that you add so they will start processing waste more quickly than having to be established naturally (see #2 above as well).
Fluval and Eheim make awesome canister filters. You'll want something that will give you a 6 to 8 fold volume per hour turnover rate (ex: a 150 gph filter for 20-25 gallons of liquid).
Edited by Dissever, 24 June 2014 - 01:55 PM.