I have about 9 RBTAs in a standard 55 with t5HO bulbs...aside from the slightly elevated nitrates, everything seems fine. I keep my salinity around 1.023 to 1.024..so yours may be a bit high...but everyone has their own. I've read that salinity should be between 1.021 and 1.025...but that's your decision.
As far as it deflating...mine deflate all the time. Some days we'll come hope and they all look like TOTAL crap! Next day..they're all looking as beautiful as always. Just give it some time and monitor your water. A partial change would be good...but don't change too much to quickly.
And it will move to where it's most comfortable. If you only have one powerhead, you may not have enough flow for him. I have a k2 and a k3 in my tank and probably don't have enough but thye all seem happy.
If you have any ?'s...PM me and i'll see what i can research.
Brock