Physically adding chemicals for a hydronic system

I need to add a bunch of antifreeze to my system (around 25gal). What do you guys use to add stuff to a system? Without getting a lot of air in there.

Clearly I need to release water as I add the chemical. So I have to find a place where the additive won't go straight out the drain. Given my layout, that is a challenge.

I don't think gravity will work out for the feed. I am thinking maybe plumbing something together using a small fountain pump with a tank and a hose connector. I can isolate the primary loops and buffer tank from upstairs so I will probably have about 6' of head with a drain opened (with a control valve to regulate flow rate). I need to see what I can find for a pump. Any suggestions.

Should I be doing something else?

When the plumbers filled my system (this was awhile ago, things got rearranged and no longer have a need for antifreeze), they put a hose on the suction side of the pump (they had installed a drain valve there), and just stuck that hose into a bottle of the antifreeze and opened the valve with the pump on...it sucked it right in. Shut it off as the bottle emptied, put a new bottle in place, open the valve until done. If you'd drained enough of the system to make room for the antifreeze, you wouldn't be throwing it away.

I only saw this once, and may have missed an important step, but that's the basics of what I saw.

Hopefully, somebody that does this on a regular basis can provide more detailed steps.

That is a good idea. I will go contemplate my system. I need to open it up anyway to replace some parts (the damn antifreeze again) and add a couple of controls to better allow me to isolate parts of the system when I need to. So it would be no big deal to add some extra pieces while I am at it.