Just a thought. Some stock tanks have a built in drain. Could it be leaking around that?
Anne

I was thinking the drain also.

Put a few drops of food coloring in the water in the filter, observe. The source of the leak will become apparent. Sandy

If its the drain, it will be easy enough to see. The soil right under the drain will be wet. Sometimes those drains just need tightened up from both sides of the tank. If you have to drain it, you might as well put more plumber's tape on the drain too.

I too had water leaking over a couple of days-I checked the drain and it was okay. After draining the stock tank I found a hairline crack going down the side 12 inches from the top. I tried several things to fix it evening using a torch to melt the cracklines together-this worked for one year-what I ended up doing was putting a pond liner inside the tank and its working fine now

Mine had a hairline crack in the bottom, too and I used JB weld for water and it is still holding after several years. Directions were to drill out the area and then mix weld and fill and so far it is still good.