I bought a couple of RES babies in China Town at the beginning of May not knowing what I was getting myself into. Same day, after a little online research, I run down to Petsmart and go out $200 to provide them with tank, basking rock and lamp, uv bulbs, water heater, themometer etc... Unfortunately one of the turtles continued to grow while the other did not. Despite my best efforts of feeding separately and providing the best care I could the smaller one died this morning.
I am wondering if it would be ok to place a store bought 4" Res or African Sideneck with the smaller turtle that is still living. The living one is very active and healthy but is probably only 2" in size so far. I'm wondering if the newer larger turtle would cause problems for the little one.
Also I want to upgrade the store bought Zilla tank setup with customized equipment. What size tank should I go with. I noticed Petsmart $1/gallon sale and was wondering if that is a smart idea or should I go with something larger from the local Mom and Pop shop?

