My fish house holds 2 30 lb tanks. Can I take one tank off and fill my 5lb tank by putting the switch halfway (use from both tanks) Will this fill my 5 lb tank? Will it fill it full or part way? maybe if the 30 lb tank was cold and the 5 lb was warm would this help?

Anyone know their propane?

I don't think that will work as the gas that goes thru your lines is vapor and what you need is liquid in the 5 lb tank. If you have a line directly between the two tanks with no regulator you may be able to transfer the liquid by turning the 30 lb upside down before opening the valves. This would be similar to filling the 1lb. bottles. I don't think this would work good tho.

That will not work. It could be done but unless you are familiar with filling propane than I would not recommend it.