I bought a 5,000 tank for water or fertilizer storage depending on my water situation at any given time.  This water is for cattle and tree/garden/chicken watering.  I will clean the tank well when going to fertilizer to water which will happen every several years.  Never mind that.

I screwed up and bought an aqua colored fertilizer tank (1.9 specific gravity capable for fertilizer) when I should have bought an ugly black one that blocked UV light.  Until I get this thing covered up with R-panel, I need to treat the water.  What can I put in it to prevent algae without harming the purpose of the water?

Can cattle consume copper without harm?  You could try that.  A few ppm might do it, so in 5,000 gallons, that's 40 grams of copper dissolved in that tank.

Otherwise, chlorine would be good.

Chlorine is your best bet. Go to a pool supply store and get the powder. You can also buy test kits there so you don't add to much or to little.

Even after you cover the tank I'd add some chlorine to it not as much as before.

I hit it with 0.5 gallons of bleach which, according to an online calculator, is 5 ppm and I'll treat any more trailer loads I add at 1 ppm.  I've read online I need to keep it between 0.5-1 ppm.

I am also going to order test strips.  Well pump is currently pumping into the tank (it times out and waits about an hour), so I'll have to monitor this thing weekly I bet.