Diamond goby acting strange. Added to the tank Thursday and everything was fine. Yesterday he was laying up the side of the tank and swam around the top but in the evening looked like he was gasping for air trying to jump out of tank. I thought his gills looked red (but I may have been wrong) so instantly done an ammonia test, I have a couple of ammonia tests so I double checked. It's not ammonia. Done a full parameter test pH was a little low, put a pH booster in. I read low pH can cause low oxygen so done a 10% water change i didnt have enough water made for a bigger change, pored it in from a height and put the wave maker at the top so it broke more of the surface water. I have a fluvel flex 123l . He was is calmer today and gills did not look red but still laying up the tank or in the corner. I was watching the tank checking on him and looking at the other fish (who seem fine) he swam hard to the top out and hit the lid floated to the bottom on his side, darted up again, hit the lid, splashed across the tank then floated down, this time he was dead. I had an ammonia spike a couple months back from making a stupid stupid error with a bit of the the stone and lost most of my fish, so i am really worried. What are some possible reasons, is there anything else i need to do? currently i have a juvenile tang, do plan on getting a bigger tank next year for when shes bigger, couple of clowns, 2 shrimp and a hermit, and a couple tinny hermits.