I’m not letting my other two fish die after nearly 7 years at all costs, advice needed

Their sibling died 2 weeks ago, ever since I’ve seen them being quite aggressive but always near eachother, no scars so I couldn’t tell whether they were aggressive and stressed or just stressed and close together

The water should be clean, it is all cleaned out, they went into the hospital tank for a fungal infection before and the water had then been cleaned again with still some from before remaining as to not shock them anyways they were fine after their hospital tank, fungal infection gone.

I’ve walked up stairs now and seen my snail chewing on two weird looking things, I thought it was just a long snorkel of hers protruding out until she slid away and it was two pieces of fin, my fish have been fighting, I’d seen them move erratically and slightly sideways and I’ve put one fish in the hospital tank to stop them from fighting I’ve dosed swim bladder treatment in, he has a filter and a bubbler.

My other fish will go in tomorrow (I’m guessing I should clean out the water and then redose it too right?) I’m incredibly scared

I’m not the primary carer here and I’m a terrible owner. I’ve recognised that the filter I told my mum was broken is in fact now broken again and isn’t working, there’s a bubbler in and they aren’t gasping for oxygen but I’m going to spend my money from either my trust fund or my savings to buy a new one.

I have to think, if my fish are suffering and are this stressed, I want them to live to their maximum capacity and as a working class person with a mum who is extremely busy and I myself am extremely busy, I might have to get them adopted? Does anyone know if that is possible and how.

I know I properly sound erratic and that’s because I’m scared. My main tank is 4 foot long and very wide we got it years ago and I sadly don’t remember the litres

The hospital tank is sadly only 19 litres, my mum chose it.