I was doing a water change this morning on my tank. I have a huge Bio-load so i do a 20% every other day and have had no problems. Well I was putting water back in with the filters turned off because the water level was to low to have them running. Well he just sank! all the way to the bottom on his side. he is a 5 inch silver BTW. so i immediately turned the filters on the canister was fine running but the HOB was not getting much water and i thought it was going to burn up but i dont care i would rather lose a $40 filter than my Aero. so i started dissolving salt into my next bucket of water as fast as i could and got the tank filled back up. (filter is fine) and he was swimming around again but acting really odd so i hit the LFS for an air pump and air stone that i have been putting off buying for some time now and when i got back he seems fine. Just like nothing happened. So i got the air stone running and i think all is well. Did i just lose oxygen in the water really fast? or is there some other explanation? I am very new to this hobby did I do everything right?