Well, this may come as a surprise, but algae is completely harmless. If you think about a RES's natural habitat, everything is covered in algae, including the turtles themselves. However, you are experiencing algae "blooms", which is a sign that your water is full of nitrates. The algae is actually doing a good thing eating those bad nitrates for you.
I am guessing you still are using that 10 gallon tank? How high do you have the water level?
My guess is that those two turtles died from a combination of factors, including coming from a parasite ridden bucket full of hundreds of baby turtles, probably sold way too young. Add in the 1 month of no heat, and you are just asking for a vareity of illnesses, from the parasite load and respritory infections.
To save your last baby turtle I would sugest getting a bigger tank, at least 20-30 gallons so you won't have to replace it quickly. Get a good filter, I suggest a canister filter, such as the fluval 104. Your setup should include a basking area, with a basking light over that spot, and a UVB flourescent tube over the rest of the tank. The easiest way I have found to set up the lights is to get a screen top for the tank, cut holes in it for the filter, and use an aquarium fixture for the flourescent tube. Fill the tank up as high as you can, the deeper the better. RES love diving and swimming, and the more water you have the longer it takes to get dirty. Don't put aquarium gravel in the bottom, RES tend to eat it and it just collects the dirt. I use river rocks and slate, some people have nothing.
If you do this setup properly it requires very little maintenance. I change water in my tank about once a year, the rest of the time I just add fresh water. My tank is 55 gallons.
Make sure you don't overfeed your turtles, if your turtle is under 2 inches it should be fed just a tiny bit everyday, no more than would fit in its head. Also introduce greens, an adult RES's main diet is veggies and greens, not protein. When your turtle hits two inches back off the feedings to every other day.
I would suggest that you not buy anymore turtles. Adult RES are the size of dinner plates, and require enormous tanks or ponds. The rule of thumb is 10 gallons of water per inch of turtle. That is gallons of water, not tank size, so a 5 inch turtle needs 50 gallons of water, in a 75 gallon tank or so. Turtles generally grow an inch or so a year, but if they are overfed they can reach 5-6 inches in under 2 years.
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A mans got to do what a mans got to do. A woman has to do what he can't.
Mom to:
1 little boy born 7/19/04
2 male RES, born 1999
1 ribbon snake, age unknown
3 FBT, ages unknown
1 female bearded dragon, born 5/2002
1 male lab mix, born 5/24/03
1 female calico cat, born 6/7/04
