I try to use a minimum of 1 liter for every 125 grams of gold filled, with a strong aquarium pump and fine bubbles. Less AP and it becomes hard to supply enough oxygen to keep it working. One gallon per pound or 450 grams or so. Two gallons will be even better.
Even better use an air lift pump and “wash” the material in a flowing stream of AP. It will speed it up considerably, but can leave an excess of cucl1 ( I think that is the correct term, white looking copper) in the solution along with the gold foils. Here is picture of a small one that can eat 1 pound of solid copper in 4 days on average. An air pump lifts the solution up through a tube using very fine holes to create tiny bubbles. It then spills over the top into the space holding the material, in this case copper bars mixed 50-50 with bronze pins. The solution then runs out the bottom through holes that are small enough to slow the flow down but not allow it run through to fast. A slight trickle over the spillway is about right. This keeps the solution from running out of O2 while in the container. This pump was $12 at our local WalMart in the aquarium section. Only the small one uses the air pump that does not sit in the solution. The larger ones won’t work as the pumps are submerged and I am not sure they will hold up to acid. Research air lift pumps online and a large number of designs can be found.
Edit for spelling.
