Bacteria bloom (white cloudy water) in water change tub?

[20 20]

My boy is getting a bacteria bloom in the water he prepares for doing water changes for his 75 gallon discus tank, and was wondering if anyone had any idea's as to what is going on. We've got a 32 gallon Brute rubber trash can that he uses. Yesterday, he emptied it out, and filled it with 65% tap water and 35% RO/DI water. He's got a power head in it for aeration. He's also got a heater in it, but had it set a bit high, water temp was 90. He keeps the discus tank at 85 to 86. This morning the water is cloudy white, the sign of a bacteria bloom. He tested the water, ammonia and nitrite are zero, nitrate is around 10. PH is 6.0. Does anyone have any idea's about what's causing this? Has anyone had a similar problem? Any help would be greatly appreciated!

[BlazerFRS]

Does your tap water contain nitrate?

If not there could be an ammonia source in the trash can that has allowed some nitrifying bacteria to take hold.

As long as it's definately a bac bloom and not some other contaminant I doubt it would be harmful to the fish... but the fact that it contains nitrates makes it of limmited use for a water change IMO