Why are Corydoras so fragile? Can you suggest me a bottom feeder that is hardy?

Can anyone suggest me a bottom dweller that is hardy? I have a 20 gallon tank. I do water changes every 10-14 days and I have real plants in it, a power filter, around 10 neons and a male and female betta. Last week, I bought 2 corydora small catfish but they didn't make it. Can anyone recommend me a hardy bottom feeder? I don't want a pleco because they poop a lot.