is your turtle lethargic (as in your turtle's head is laying on the basking platform, literally)? basking all day isnt normal, but i suppose you could talk to the vet and see if it might be a side effect or just characteristic of a recovering turtle. for my turtle, this was a definite sign of something wrong (started frothing at the mouth and bubbling out of water; made scary sounds). also, you could put a sponge on the filter to slow the current down, maybe it would help your turtle swim. when was the last time your turtle ate?
my RES's first night on antibiotics was much better than when we found him wheezing that morning, although the symptoms did not fully subside. just let the antibiotics work overnight, you're doing all you can. good luck, keep us updated!
