New fish vet in Bristol to note - operating on a molly (not goldfish despite headline) - I picked up the paper on the tube

The clown loach needs company and should get really big, so unless you are getting back into this in a very big way, might be worth seeing if you can find anyone to rehome it to or whether LFS will take it.
If you post some photos of the others, people can have fun trying to ID

THIS is very good sinking pellet food
What sort of goldfish are they - single tails or double tails, egg shaped or slim?
How are they now?

It is a bit of a shock!!

He's gorgeous.
Be careful with that help, on one memorably bad day my Maine coon (now sensible adult cat of course) went swimming in a tank of danios then spilt enough Prime to dechlorinate an Olympic swimming pool ...

Can you order a tank divider, something like THIS
Always useful to have ...

When you say you were due a change of filter sponges, can you elaborate? Is this a filter with cartridge system sponges with e.g. carbon in, or are they ordinary sponges? Carbon sponges do become exhausted and have to be changed frequently - but really there is no need for routine carbon unless e.g. your water is getting stained by tannins from bogwood and you don't like the colour - you could save carbon sponge for use only if e.g. you need to remove medication and use the space for an ordinary sponge.
If this is just an ordinary sponge, normally you would not change it until it looks worn out and might start to break down - and at that stage you can change it if it is not the only sponge. If it is the only sponge you should cut in half and change the second half a few weeks later. When the sponge starts to look clogged you should just rinse it out a bit in old tank water when doing a water change, then put it back in filter.
It is disappointing but not unusual when shop fails to give proper advice

Dr Tim's aquatic ammonium chloride - yes that's the right sort of stuff, not too expensive on Amazon - about a fiver inc postage - thanks for the info, had not seen this previously

The alternative if anyone can't get hold of pure cleaning ammonia is this stuff BIOMATURE which is really just posh ammonia. You can ring them up and get it posted out. You must follow their instructions strictly about dosing, and you may need to buy their Nitrite test kit if yours does not measure high enough, but it is a fairly idiot-proof means of fishless cycling, although a bit more expensive.

Says maybe ...