Sounds like a combination of issues that may have led
to the fishs' demise. If you have properly aged your new SW
prior to use (12-24 hrs) and your top off water is not killing your pH, I would relate this to husbandry. The water change otherwise sounds coincidental. The water quality is well below par which quite honestly impairs the fish's immune response as does the stress of overcrowding. Combine that with the possible use of FW
(?) feeder fish, and you have a recipe for disaster in regards to the lion. Feeder fish in general are not a good choice for many many reasons SW
. The fattiness of the FW
fish can harm the lion over time and cause internal bacterial infections. The SW
feeders can introduce a host of problems common to all SW
marine teleost species and unless raised by you or properly QT
'd and cleared of any common ailments not a good choice, even then with caution. The jaw gaping could also suggest lack of a proper diet (goiter) which can also be a direct result of the food types fed.
Overall, I think you really need to re evaluate your stocking choices and properly research the species you wish to keep before getting them. Sorry if this seems like a lecture but I don't know another way of phrasing this so you understand it's importance.