Hi Luna, first thing I would do is check your water parameters and temperature in case they've gotten out of hand whilst you were away.
What kind of filter do you have, does it have a strong current ? All these things can contribute to gill damage.
Also, those stones look like they'd trap a lot of food and waste under them, must be a nightmare to clean - I'd recommend removing them and replacing with fine play sand (thoroughly washed of course) or have the tank bare bottom.
Post some water readings, temp etc for everyone so we can help you out :happy: