You said use the 12 on a 20 gallon tankYour tank is way too small. It doesn't look like a 20 it looks like a 10 gallon. Personally I would get a large storage tote for now if you can't afford a bigger tank. I would also just get the arcadia set that petsmart sells in store now. It'll come with the 12% which is too strong for your small tank but will work with a large storage tote plus your bigger tank when you can afford it.