While I'm on about quarantine tanks i'll explain my basic set up which has served me for years.
Its a jewel 125 which has had more silicon over the years than all of red seas 750xxl put together, but its a qt so doesn't have to look pretty.
I only fill it with 60 litres of water, no point filling it to max 125 litres, it just uses more medication if needed so its half full.
I use a Fluval U3 internal filter and a small powerhead for surface agitation, a small 150w heater, Seachem ammonia alert and a cheap Chinese light. That's it for equipment but I have a couple of grey pipe fittings for the fish to hide in.
I always have 4 sponges in the sump (U3 uses 2) and when a fish has gone through quarantine I clean the tank thoroughly, fill it with fresh water and replace the sponges with new ones from the sump and put 2 more sponges in the sump so don't need to cycle tank every time, I buy a large square of sponge and cut to size, no point wasting money on branded sponges.
I keep the fish in qt for 6-8 weeks without medication, I do not see the need to medicate and put the fish through stress if its not needed. I don't agree with prophylactic treating of fish as its unnecessary. Would you treat yourself with a bunch of medication if you didn't need them?
If the fish need medicating I keep them in qt for 12 weeks or until 100% sure they are cured of whatever illness they had.
That is my quarantining method which I have always done and never had a disease in my dt.
I did have a tank crash about 15 years ago which killed my entire tank but that was due to buying a fish I thought was bullet proof and i put it straight into dt and within a week every fish was dead.
A qt doesn't have to be expensive, a cheap and cheerful basic set up is enough for a qt.