Oh you're in Canada, that stinks, I was going to offer to take the goldfish to live in my pond if you were closer! 40 gallons would work fine for the goldfish and pleco (until he gets bigger, IF he is a common pleco). You should do a google search and see if you can match the pleco up to a specific type so you know if you need to make arrangements for him at some point. I don't know anything about tetras though, so I can't help you there! But I did a search and I read that tetras can live in 70-80F temperature. So they will probably be fine, as long as its not under 70.