You have something growing in some corners etc that needs to be taken care of. I would shock the problem tank when it was full of water with a high concentration of bleach (=chlorine like a swimming pool) maybe 5 gal in a 100 gallon tank, stir it to mix and keep the cattle away from it for a day or so.

 Then empty, scrub, rinse thoroughly and refill with clean water with adding just a couple tablespoons of bleach per 100 gal of water. This is what is recommended for RV water holding tanks. A "little" chlorine in the water won't hurt the cattle and will help keep it clean.

Just keep the cattle away from the tank when it is in the shock concentration. I did this in an automatic waterer and it works.

Jim at Dawn