Good cheap Fish for a bowl?

so I returned my koi and fantail that were living in a 3 gallon bowl. The same idiot who sold them to me offered me something called a sicklid or something, dont think I spelled it right. It was a convict sicklid I believe. Since i dont trust him anymore, what do you guys think? Should I get this sicklid thing when i go back tomorrow, or if not, what? And what can go in a 30 gallon with 2 7" clown knives? It looks empty because they hide behind a rock. and they wont get too big, they grow to tank size