Most here use the liquid API kit Dela linked.

Second cat fish looks like a common 'marble' plecostomus...
They get pretty big and are big waste producers.
He may not look well since I see no driftwood in the tank???
They eat wood and fish food more then algae...
The two cats would be best going to a larger tank(possibly trade to
LFS)...
A bushy nose(BN) plecostomus is a smaller and mayhelp you with water quality vs.two big cats.
Niether are eating algae of any worth for you or the tank
IMO...