Do you have any fish in the 20 gallon?
No matter what, you are going to have to buy the test kit..I know they seem expensive but you really can't know what your tank is doing without it.
Without knowing where the 20 gallon is at, I would not subject the fish to the cycle.
I would CAREFULLY, with the vac, dig it deep in an area down into the gravel without swishing it around, let it suck until it runs about clear, then move over. With only 5 gallons, you won't be able to do much at a time. Each day, do a little more gravel. You may have to do a 40-50% water change in the morning and the night.
The best chance your fish will have is if you make sure that water as as clear as possible without any chance of getting nasty.
Also, I would take out as much gravel as possible, leaving a thin layer of maybe 1/4" for the pleco. You need to get as much water in that tank as possible.
You also need to feed very sparingly for right now, just a little in the morning, etc don't feed two or 3 times a day.
Do you feed the pleco different things than the fish? How big is the fish?
how far is the 20 into the cycle?
