Sanitizing fresh water tanks, especially for fish, is not difficult. You can use items commonly found in a bathroom to clean and freshen up a tank while the fish are in a different bowl or container. Hydrogen peroxide, or H2O2, is used much like bleach, but is safer to use. It also kills most forms of algae. You will only need to clean with hydrogen peroxide every few months and it takes just a few minutes.
Things You'll Need
Syringe medicine dropper
Fresh water tank
Using a medicine dropper syringe, squirt hydrogen peroxide into the fresh water tank, aiming for sections that are dirty or covered with algae. If you can't reach them all, that is fine, the peroxide will disperse in the water and reach those areas. You shouldn't put in more than 1 mL of peroxide per gallon of water, so it is not a lot of peroxide.
Leave the peroxide in the water for at least 15 minutes.
Change out the water like you normally would, leaving about a quarter of the old water in the tank.
Repeat if necessary after a week if all the algae has not disappeared.
It is possible that more fragile fish may be negatively affected by the peroxide, so if you have another tank you can put those fish in, you can.
The algae and dirty parts of the fish tank will start to bubble when hit with the peroxide, this is normal. The fish may gravitate towards it and start to eat the loose algae. That is fine, as well.