It’s a 20G long. I think I’ll forget about guppies.For anyone who wants to keep guppies, buy a group of females and put them in a tank with hard alkaline water, add a heap of salt and keep the salt in the tank for 4 weeks, then dilute it out. Deworm the fish during this time. Let the females breed and produce a heap of babies. Get rid of the adults and keep the young. Grow the babies up and you will have some healthy guppies.
A 20 litre tank is a bit small for all those fish.