Molten aluminium has an enormous affinity for hydrogen - it dissolves hydrogen very easily. If you have moisture present (H2O) then the molten aluminium will take up the hydrogen and release it as it cools. If you do a melt on a humid day the problem gets worse. All tools, flasks, crucibles etc that come near the melt need to be pre-heated, as does the ingot or scrap that you are melting.
The dissolved hydrogen can be removed by 'scrubbing' with a gas. Several methods are available. You can get purple tablets to plunge in the melt, that give of a gas that removes the hydrogen:
I use an argon lance. This uses the gas from my TIG welder, applied via a length of stainless steel 15mm water pipe, which I have crimped at the end and drilled radial 1mm holes to form small bubbles. I previously used dry nitrogen which also works, but was able to give back the rental cylinder when I got the argon for the TIG.