You put it in till it's full, and if it only takes one tube then great, but if you're having problems it's probably going to take 2. I put it in because there wasn't enough resistance and the fan would freewheel too easily. With engine off, if you grab the fan and try spinning it, it should barely turn before coming to a stop. If you do this and it just keeps spinning you need fluid. Pretty simple really, fluid offers resistance. If this doesn't solve your problem then you have a cooling system problem and need to get it fixed.
