I'm afraid 5.5 gallons is too small for all the fish you have in there; and they are all shoaling fish which need to be in groups of at least 6 each. They will be very stressed in this tank which is probably why they are not eating.
None of the fish in the 20 gallon can go in the 5.5 gallon either.
It would be better to move all the fish in the 5.5 gallon into the 20, but that would leave the tank overstocked and poorly stocked.
I know you don't want to hear this, but for the sake of the fish I need to explain.
You have many shoaling species - the cherry barbs, all the tetras, the otocinclus and the kuhli loaches - which need to be in groups of at least 6 of the same species. You don't have room for 6 each of all these fish.
The hillstream loach needs fast flowing water which is cooler than the other fish need.
Guppies are hard water fish while the others are all soft water fish, though guppies may be OK unless your water is very soft.
The honey gourami has no problems.
You have 2 choices. Either upgrade to a bigger tank and increase the numbers of the shoaling fish. Or rehome some of the fish and increase the numbers of the others.
A betta would be the best option for the 5.5 galllon.