go to plumbing shop, tell them what you want, you want a small float valve, drill a hole in the top side of your water tank the same size as your float valve , ball cock, , you will have to do it near the opening to get your arm in and out of tank to fit it,
fit the valve inside tank coming out of drilled hole, with the nut you got with it screw it from the outside, bingo
if you dont understand ill send a couple of pics when i download them
just get the right drill attachment for the hole size its called a FLAT WOOD BIT, aLDI HAVE SOME IN , STILL ON THE SHELVES UP HERE