Aluminum electrolysis: inside storage tank of a boat

I have a 1999, 36' Doral. It's located in lake St. Clair Michigan. (Fresh water)

I noticed some matter in the bilge while winterizing this year. It looked like it came from the Head storage tank. After pumping out the tank I removed it. Once outside in the sunshine I was surprised to find all along the 4' weld seam on the bottom leading edge of the tank was about 6 what best are described as rot holes. in an Aluminum tank. Now the bottom edge is angled so you have a leading angle sticking down in the bilge. So if water was down there that is the part that would be wet.

I have never heard of an aluminium waste storage tanks rotting thru due to waste? The boat is 10 years old and I personally put it up each year, so I know that tank is 99.5% empty. It's only used 4.5 months per year. Otherwise it's winterized.

Is it possible that that edge is under going electrolysis? It has a gage connected to it so power is going to it. And if water is in the bottom of the tank inside or out would it would rot?

If the answer is yes, should I then be worried about my two 70 gallon gas tanks? Nothing there as of yet? No fumes as of yet.