is my turtle sufficating?

Question
i have a red eared slider turtle it lives in a 40 gallon tank i only fill it in half way.i have a water heater that keeps water at 75 degrees ihave a uva lamp that heats at 85.still saving for uvb lighting.i feed my turtle a veriety of pellet foods and one fish a day.it doesnt stop sheding you cant even see the brown much.it opens its mouth as yawning constantly and whight material spits out sometimes when breathing.it also makes weired noises when breathing what should i doplease help.

Answer
See a vet for one. Nothing I can tell you here will help you save this turtle without you seeing a vet first. The turtle likely has a respiratory infection or worse. The discharge from the mouth is common in severe lung infections. The continuous shedding could be a condition called SCUD, which can also be fatal. He should probably get enrofloxacin @ 10mg/kg q.24h PO. Take that recommendation to the vet.

Next, you need a UVB light and there is no excuse for having a reptile before you got the proper light first. The scutes falling off could be an indication of MBD and/or renal failure. MBD can be caused by lack of UVB, not to mention you causing deformities in him as he grows by not having one. In effect, you are killing him slowly by not having the proper lighting. Their marginal scutes fall off in severe renal failure, and this may be happening as a result of complications from MBD, or some other cause, such as an acute toxic exposure to something.

You did not state that you had a dry basking area for him, or what the basking area temp was. He needs a dry basking area, and it should be warmed to 84-86F. Having a wet shell all the time, with no opportunity for him to bask and dry it off, along with dirty tank conditions, can lead to SCUD/shell rot. You didn't state whether you had filtration, so I recommend filtration that is rated for 1.5-2 X the size of your tank. In other words, for a 40gal., you need to have a filter system rated for 60-80gal.

Lastly, you didn't say what kind of fish you are feeding, so I am going to recommend you knock off the fish until you do more research. Many types of feeder fish, in excessive amounts, can lead to thiaminase poisoning. This may be to blame for his sloughing scutes. One fish a day, and feeding every day for that matter, is excessive. Most turtles need only be fed every other day unless they are hatchlings.