WATER STORAGE TANKS

I assume the tanks would be filled to within a few inches of the top (leaving room for thermal expansion). Seems like a short dowel with a nail drilled through near the top to keep it from dropping in should do the job. It only needs to be long enough to get a little below the desired level. No water, add some more.

A string with a nut on the end?

Is this a trick question?

Black rubber hose works great, easy to see.

i will be filling my tank soon. do you not fill it as full as you can and then let the autofill completely fill as the air gets purged from all pipes and lines?

Yes, for pressurized storage. Well, actually, I don't use my autofill stuff. I just run a hose to a hose bib on the bottom of my system & fill there, run another hose out of a top bib to let air out & so you'll know when it's full. Then after it's full & you're sure there's no leaks, keep the water going until the psi gets to where you want it.

But this OP has an open unpressurized tank.

ahhhh 10-4. good thinking on the bibs top and bottom. i have a stainless tank/water truck coming to deliver so hoping to make something up to mate the two... havent seen the truck/hose yet.

just figured where my autofill is in my existing system once i open the valves to join the two on initial firing the autofill would do the job of topping off at desired pressure. i will have to circulate to get the air out of my underground lines and such.