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Hi, yesterday my 1 and a half inch res was opening its mouth ( assuming it was breathing ) what are other reasons that it opens its mouth??? is this serious?? it eats just fine, is it sick?

Usually gaping (or opening the mouth) is a sign of a respiratory infection. RIs can only be treated by antibiotics from a vet so it's best to make an appointment as soon as possible, the longer you wait, the less the chance for recovery.
The fact that he's still eating is a good sign that he'll be strong enough to fight the infection so make an appointment asap.

Did he yawn or was he breathing with his mouth open?

You said he was opening his mouth yesterday---how's he been today? He was opening his mouth while out of the water? He has no other symptoms that would indicate he has an RI? If not, then I'd keep an eye on him. If you notice other abnormal behavior (or if he's still doing it), then have him seen by a herp vet.

What are the water and basking area temps?

mine yawns sometimes out of water before bed time its cute, but if its not constant i wouldnt think its an issue?

when in the water, he could be just taking in some water

to marisa: I havent seen him opening his mouth today, but the last two days, it opened its mouth twice in a row then another time it was a bigger one ( it was when it was out of water ) the water temp is 78 and basking area is 90. I will get him to a vet soon for a check up. I've called the vet near by and they say he was going to be back in 2 weeks well today my res seem to doing ok, no yawning, eating fine, and he does do alot of bubbles in the water ( not sure if he is playing around ) i read somewhere on this forum that if the bubble doesnt pop it would be a sign of RI?

if he does have RI after getting check by the vet he would be ok after that right? but RI could come back again correct?