hilandfrog wrote:
...........Buy a water proof hatch cover, 5-6" (one with o ring) from boating store.
Cut required size hole in fresh H2O tank,
Mount hatch cover,
Open hatch reach inside and clean out tank at the end of each season........
X2. I don't think you can get rid of growth once it takes hold without scrubbing the interior of the tank. I recently had the same question and after speaking with a fella who had the same problem on his sailboat, he purchased an access hatch from West Marine and installed it in the top of his tank. Since you probably can't access the top of your tank, I recommend putting the hatch on the side of the tank. I would hesitate to put it on the bottom.