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I have a 50gal tank with some cichlids, 2 severums and a loach

The water is deep but i'll make the appropriate basking stand out of the water etc

Could i add one baby RES to this tank

That size tank would be good for a res up to 5 inches, however, he or she will eat any other inhabitants of the tank.. something you may not appreciate...

If you care about your fish, then no. Like megcornell said, they could become food. Over time, that tank will be too small for the needs of your turtle as well.

Depending on the size of those creatures, it sounds like it could become a heavy bioload rather quickly. I think a baby RES would do better in a tank all it's own.