Oh, geez, I'm sorry your pleco died. You need to read up on the "before you get fish" at the top of this newbie forum. It seems like you set up a new tank without cycling it? That is very important to grow the beneficial bacteria the fish need to be healthy & survive. You also need to learn what maintenance & how often to keep your fish alive & healthy.
So you have a beta in a 5g. How do you care for him? How much & how often do you change his water? What do you feed him & how often?
So how are your catfish doing? I think, since your tank is NOT cycled, you need to change water every day to help them stay healthy. I would try for at least 50% if you can but no less than 30 or 40% EVERY DAY!!! You need your own at home water test kit & do it until you have "safe" test numbers (NOT a store's version of safe!). We can help with those safe numbers but you have to test & post here what they are...every day!!!!
Sorry, it's getting late for me to help tonight. Maybe others will have more suggestions but change water often!! & Test!