I need help!

[Linzertoonie]

Hello Everyone,

Hoping someone can help me figure out what's going on with my fish...
I have a 17 year old Comet that I recently moved to a 55g planted aquarium.
He's been the only fish for a long time because I only had a small tank and moved around a bunch. So getting back into the hobby, I've had my 55g running for 3-4 months and everything's been going great.
Added some plants.. Anubias, java fern (2 types), and some others.
I added some Cories to the tank along with a small bristlenose pleco, a Featherfin catfish, an Albino Rainbow Shark, a panda loach, 2 Apistogrammies and some Tetras.
Oh and also some Trumpet snails and a long eared snail.
We have had a heat wave here lately so the temperature of the tank has gone from its usual 24 degrees Celsius to 25.5-27..
I recently had 2 Cories die in the last week.. I couldn't see anything visibly wrong with them other then a few days before they were breathing faster and laying on the bottom not doing much.
My Albino rainbow shark has also been acting lethargic for about a week or so.. looking like he's lost weight and the bottom of his tail looks strange.. he's been hiding in the plants and haven't seen him do much.. looked a bit better yesterday but today back to the way he was..
The shark was totally anorexic when I got him but put on weight and has been thriving since the last week.
I've been doing 30-45% water changes about once a week and vacuuming the sand.
And the water parameters have been ok..

The last I checked:
ph: 7.6
Ammonia: 0
Nitrites: 0
Nitrates: 10-15

Also I fear my Comet is not loving the temp increase. For the last week or so he will freak out and fly around the tank bumping into the glass and other objects.. he is blind as well.
But something is definitely bothering him.
He looked like he injured his chin last week and it got better but now more of his chin is looking bad and I'm not sure if it is from hurting himself while freaking out, or if he has a disease??
I have seen him gape his mouth open a few times and i just saw one of the Apistogrammies gape his mouth too but maybe that was nothing, but I did see him rub against the sand a few times.. maybe parasites ?

The newest things to the tank before this started happening were the long eared snail, one Apistogrammy and some new plants.

I have gotten the temp down and holding around 25 or just over 25.. but the Comet is still freaking out...

Any help and suggestions would be great!!
I'll add some pics of the shark and the Comet.. hard to capture his chin though..

Lindsay

[mimiheart]

Do you have any oxygen/airstones running in the tank? Higher temps mean less oxygen, which is why goldfish do better in colder water.

Those aren't really what I'd consider compatible tank mates. The only fish goldfish really do well with are other goldfish. The shark will like things like algae wafers and sinking pellets and may pick on the goldfish at night. It may have also picked on the cories.

All in all, though, the 55 is probably just big enough for the comet alone. Especially at that age. While he may like some friends, he'd be best with other goldfish around his size.

[Linzertoonie]

Yes I have an air stone and then a long bubbler as well.
This shark is small and 1/3rd the size of the goldfish, and isn't aggressive at all.. or yet anyways
I know it's not the ideal situation .. seems like there's enough room right now?
So you think it's an oxygen problem or all the above ?