If yoyu plan on removing the plug for draining later, you could try teflon paste. It is found in the plumbing section.
You could also use a polyurethene sealant marine grade such as sikaflex or a black poly roof sealent.
I have had good luck with all the above.

try teflon tape. Simply wrap a few turns of the tape around the threads and re-install the drain plug. Works great!

I agree with the teflon/plumbers tape.. Should work great and you can also remove if needed. Glue might not let you do that down the road.

Thanks everyone,
I emptied one of the tanks out and investigated a little closer. I think where it is leaking is from behind that big round part that is behind the square plug. I didn't realize that that part needs tightening too. I tightened that up, and I will add some more of that plumbers tape to the plug.
Now that I think about it, surely that big round part behind the plug must come out too, and I could put some tape on that. But it didn't seem to want to come apart.
I shouldn't tighten plastic things too tight, right?
It would be great if this worked. I had several stocktanks crack over the winter, and I repaired them with "pipeweld", and they seem to be holding. Now if I can fix this, I'll be back in business! Thanks again for your help!

Cathy, I just knew you would have success in your endeavours with the tank repairs.
Nothing like ignoring the sceptics, myth makers and gloom and doom merchants and giving something new a try.
So what the hey, if you had not tried it you would never have known for sure whether it would work or not.

The part you are talking about may not screw out at all, it may have been a pressure fit to start with???
Maybe someone with a plastic stock-tank would know just what part of the drain assembly you are talking about and come to your rescue.
It maybe a case where you could put a ring of the plastic weld around it and give it a shot with the heat gun.
Now you are a plastic welding expert......anything is possible! LOL
"Horton"

Hi Horton!
Thanks for all your help and encouragement with my stocktanks! And thanks for telling me about the pipeweld stuff.
I think I've seen replacement drains, so they must come apart somehow.....but hopefully all mine needed was some tightening. My frogs are getting impatient, standing around tapping their feet, waiting for their favorite places to be up and running. :)

If you use teflon tape, sometime try teflon paste. It works much better than the tape.

Thanks 1sst.

I stopped mine leaking with a special plug from the hardware store. The plug is ordinarily used in the drain plug of fishing boats. It is made of rubber but has a lever and screw device inside of it so that after you insert it into the hole you turn the lever and then snap the lever down to enlarge the plus and seal the hole. Cost is about $6.

Hope this helps.

Charlie

Thanks Charlie!

I learn something new everyday. Is it possible that the tank has warped? That could cause the leak in the plug. Please don't try to take the square piece out of the round piece. It's there to give you a better grip when the round piece has to be turned. Sleepless

Both of my Rubber Maid stock tanks leaked also. Ya gotta tighten the dog snot out of them plugs plus teflon tape with pipe dope..yes my humble opinion.

Several years ago, when I got these, they leaked there too...but it eventually stopped. I think the gunk in the tanks slowly sealed it off.
My bigger stocktanks that I put in the ground, we sealed with caulk. Gosh, I hope we tightened them up first! :o