What was wrong with my fish. How to sterilise tank and equipment to re use

I had 2 separate tanks with 2 goldfish in 1 and a single 1 in another. I used the same hose on both for water changes. My ranchu had a hole in head that was red with a white middle. I treated this with waterlife and it went away. Then both fish started to get fluid around the eye sockets and both their eyes started protruding out (popeye). 1 of them also had a red streak on the tail. I went to the vet (I live in UK) they gave me Baytril antibiotic. The fish were lethargic, occassionally rubbing on the sand and then started refusing food. On the 4th day of antibiotics I woke to discover they had all died. I think it was triggered by stress for the 2 (and the other single fish was caused via cross contamination by myself). I recently upgraded them to new tanks and external filters, I want to be able to use them again in the future and was wondering if anyone could give any insight on what was wrong with them and how I can properly sterilise the equipment as I spent a lot of money on it and never want this to happen again as I am absolutely devastated.
Water levels were all good (I use apI master kit), the fish were originally from a pet shop

 

You could nuke the tanks and equipment with hydrogen peroxide.

Peroxide How To | Freshwater Fish Disease 382067

 

You could nuke the tanks and equipment with hydrogen peroxide.

Peroxide How To | Freshwater Fish Disease 382067

I was worried that they had fish TB but I'm not sure; would hydrogen peroxide kill that?

 

It will kill any bacteria including your beneficial bacteria. Works similar to bleach but without the need to rinse so much.

 

I have 4 tanks of daphnia for feeding. Tanks crash every few weeks for whatever reason.
I drain, scrub with vinegar, rinse, fill with 1/20 of household bleach including sponge filters (stuff grows you wouldn't believe) for an hour, rinse, fill with triple dose of Prime then fill with display tank water and start a new culture from the cleanest daphnia tank. Dry between each step.
I can't imagine anything including Aliens surviving that.