Our vacation home uses rainwater from the cistern to fill the toilet tanks. The cistern water is untreated. (The main use of the cistern is for irrigation.) The toilet tanks have algae covering the top of the water surface when we return after a few months away. What can we put into the tanks when we closeup the house that won't damage the Toto GMax toilet mechanisms, yet will still eliminate the algae?
Vinegar did not work. I'm considering a non-chlorine bleach like a oxy-whatever -- any previous experiences with issue?
My catalog of algaecides contains minimum 1 gallon quantities for about 500,000 gallons of water, for $55.
Try geting a small amount of copper sulfate and add a small pinch to the water in the toilet tank. If you put in enough to get about 10 mg of copper (I estimate 1 mg/liter) in a typical toilet tank that should be enough to take care of the algae.
Another guideline would be to make it 10 times as strong as recommended for an aquarium.
The amount you need is probably too small to measure with anything you are likely to have. However, an overdose isn't going to hurt the hardware in the toilet tank.
You don't want to make the dose so high that it will kill the bugs in the septic tank.
You might also consider treating your cistern. I suspect that it has a heavy infestation. Copper sulfate is better than chlorine for that application (non-potable) because it is persistent (will stay there).
You may be able to get copper sulfate at an aquarium supply place.