I can't for the life of me keep plecos alive in my tanks. I haven't had an issue with any other fish, but every single plec I've purchased has died within days. Something about my water simply doesn't agree with them.

If your water levels are good, I would recommend upping your corydora numbers (they do better in groups of six or more) and getting rid of the glofish (too active for your tank). For the algae, do you have a planted tank? if you find the right balance between light, CO2 and nutrients, it won't be a problem.