Why are my fish dying? UREGENTLY NEED HELP!!?

Suprbab3 Asked: Why are my fish dying? UREGENTLY NEED HELP!!?

We have had a stable aquarium of 4 guppies, 3 cherry barbs and 1 dwarf gourami for some time.
We are new to fish keeping, so 2 of our fish did sadly die a few months ago but they are all we have lost in the past 6 months of owning our tank.
Recently, our gourami and one of our guppies were acting strangely for a few weeks, then suddenly in the last few days 3 of our guppies have suddenly begun floating around at the top of the tank or laying at the bottom of it, swimming weakly and 1 of them has died, the gourami has simply laid basically lifeless (but not yet dead) at the bottom of our tank. We moved the gourami and 2 remaining ill guppies to a 'hospital tank' leaving only 1 guppy and our 3 cherry barbs healthy.
1 guppy has swelled HUGELY, which we think is swim bladder. However, the other 2 guppies (including the dead one) began to develop some kind of black colouring on their fins/tails and bodies, as well as their fins/tails becoming frayed and torn-looking, and the gourami has also started to go a dark almost black colour where it used to be a lovely bright blue.
At first I thought it may be fin rot, but all other fish we've had (cold water mind you) that had fin rot got white on them not black, I've looked on the internet and the pH of our tank is stable so it's nothing to do with that;I'm just completely stuck. Can anyone help me at all? It's very upsetting to see our very beautiful fish suffer and die like this
Thankyou for any help.