Any idea why my fish are doing this?

I just moved & also changed the water in their tank. They are all congregating in one small corner by the filter & the air bubbles. It's much hotter here by like 15 to 20 degrees. Do you think it's that? Why would they all want to be there? It's a 55 gallon tank & right now I have 6 various types... show more I just moved & also changed the water in their tank. They are all congregating in one small corner by the filter & the air bubbles. It's much hotter here by like 15 to 20 degrees. Do you think it's that? Why would they all want to be there? It's a 55 gallon tank & right now I have 6 various types of goldfish & 1 catfish.

My husband has been putting an icebag in there daily just in case the weather here is too hot for them. Any suggestions?
Update: Ooh. How do I oxygenate the tank? Should I leave the top flaps open where the lights are?
Update 2: 8 in the corner . . . it's actually in the 90's here. We were around 70-75 degrees at our old place by the beach. Is 90 plus really bad for them? We have A/C but don't run it 24/7. That's why we're putting the icebag in there.
Update 3: WOW! Thanks everyone. I have more air stones & another pump that I'll put in & I will definitely do more to cool down the water. Thank you!!!
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