Treating the water in a portable evaporative cooler water tank

Hi,

I have an Arctic Cove 500 CFM portable swamp cooler.

The owners manual states that every time I am not using the cooler I must drain all of the water out of it to discourage undesirable bacteria growth.

Well the unit is heavy and it will be a real pain to always have to drain the water out ever time. So I was wondering if I could treat the water instead and drain it less frequently. I have researched the topic online and I have got contradictory information.

I read that I can treat the water with one teaspoon of household bleach per gallon of water, but then someone else said doing that would damage the cooler.

I also searched Amazon for a commercially available water treatment product but couldn't find any specifically for swamp coolers but did find some treatments for humidifiers.

So can anyone help me with this? What about bleach? What about some commercial product I don't know of, or could I concoct a homemade product that would do the trick? Or is there no solution other than draining the water every time?

Thanks very much.

John