I use bleach in mine every week. Use just a little amount, swish it around and you dont even have to scrub hard at all to get all the gunk and slime out of it. Rinse very well and you will be very happy with the results.
I mix bleach and water in a spray bottle. Dump the tank spray out with the hose. Then spray the inside with the spray bottle let air dry. Rinse and refill. I put a cap full of bleach in the tank after I fill it. They are 20 gallon tanks.
I use pinesol. I've used it for years and it work great. Put a couple of splashes into your tank, mix with water and take your brush and wash it. Rinse really well and you have a nice clean and disinfected tank.