Bully Trouble

It could be that goldie is sick. It is natural for fish to pick on and even kill a sick fish - this is simply because sick fish attract predators, something healthy fish don't want around in case they too get eaten.

Do you still have the 20 litre tank that you can set up for goldie? That would be a better place to keep him and observe him.

I'm assuming they're the white cloud mountain minnows as they're the ones more readily available.  I had a similar situation arise with mine.  I moved my betta out of their shared 60 l aquarium for a brief period.  While the betta (who had been aquarium boss) was out of the tank the minnows coloured up with really popping red fins, darker grey bodies and a more pronounced white stripe.  When I returned the betta to the aquarium they let the betta know he wasn't in charge anymore.  Fortunately the betta didn't get ill from it.  So what's happening in the 80 l is a redefining of territory and position in the aquarium.

Adding some dense plants if you haven't got any will help.  Increasing both platies and minnows to seven each will sort out the aggression too especially if some female platies are included.  They won't allow themselves to be bullied in front of the girls.  In turn both platies and minnows will be more concerned with their places in their own groups rather than battling each other.

As Sue says it looks like goldie may have got sick from the stress of being chased.  Something gentle like eSha 2000 can be used while you keep an eye on him for other signs of maybe a bigger problem.

The guppies are being male guppies. Are they biting chunks out of silver or trying to mate with him? The latter is typical of male guppies; in the absence of female guppies they'll try to mate with every other fish in the tank regardless of gender and it looks as thopugh silver might be the target for their behaviour. Perhaps they see the larger platy as being a more deirable mate than smaller minnows ie bigger, healthier, more likely to propagate their genes despite the fact he's male.

You could try getting more platies, that would spread any harassment out a bit but it won't stop it. I would also refrain from getting females of either species or you'll be knee deep in fry in next to no time.

The only other option is to rehome either the platies or the guppies.

Yeah, platies and guppies breed like crazy.  They eat their own fry though so if you're OK with the circle of life that shouldn't be too much of a problem.  Otherwise Sue has the only solution. 

With plenty hiding places some fry will survive but most will be eaten.

Don't forget you need at least 2 female guppies for every male so that means you'll need to get at least 14 females, which will make a huge amount of fry between them, and if only 1% survive, that's still a lot. You've already witnessed what will happen to the female guppies if you don't get 14+ 

Yesterday I added two more male platys to the main tank, one of which is now swimming happy with the silver platy and together they are keeping the guppies at bay so that issue is solved. The second new platy is staying in hiding, and goldie is still alive in the small tank but still not swimming much.

I'm now wondering if his behaviour is due to stress caused by only having the two platys in the tank together as they like to be in groups.

I was thinking I might move him into a floating nursery container in the main tank and put the more timid of the new platys with him to see if he perks up.

The hope wiuld be that the two pair up like the other two have.

what do you think?

Oooo a poll thingy, that's new.
Shame there's not a "I'm clueless on platies" button.
I could view it as a multiple guess choice exam paper and try to rule out the obviously wrong one. Nope that didn't work ..... phone a friend?