just because sterlet are small sturgeon type fish, that doesnt mean theyre a home aquaria type fish. Considering theyre used to large bodies of water, if you want to house a sterlet or any kind of sturgeon think it over long and hard, and if you want to invest that much into a fish, you need to provide it with a good sized tank (by its standards, not yours) and if you can do that, you can afford a chiller for sure.
Im completely against keeping sturgeon outside of HUGE tanks and commercial setups