Bubbles in the coolant tank

You shouldn't have to add any antifreeze at all. There shouldn't be any bubbles either. Something is wrong. Pull the oil dipstick & see if there's any antifreeze on it. If so you have a cracked block, warped head, or blown head gasket. Any of them are big dollar repairs.

I would start with a basic cooling system pressure check. If you have a leak, or loss of coolant externally, there could be air pushed back into the reservoir when the pump spins up. The bubbles can be caused by the air purging through the system, the reservoir(surge tank) is often higher in the cooling system layout. A loss of fluid due to a leak would create an air pocket that would not purge until coolant is circulated. Assuming that the vehicle did not freeze, nor is overheating, might eliminate an internal leak. A secondary test for internal leaks is a block test that uses a special fluid, in a special tester, that pulls air from the closed cooling system through the fluid. Any presence of hydrocarbons(products of combustion) causes the fluid to change colors, usually from blue to green or yellow depending on the severity. This testers are available at NAPA, and your major tool dealers(snap on, MAC, MATCO), i'm not sure who else has them. The block tester is pretty reliable at detecting internal leaks. There are circumstances where an internal leak can exist and not be detected, like a cracked intake manifold, leaking intake gasket, etc..., situations where combustion pressure is not present, or higher than cooling system pressure.

Monthly fluid loss in normal driving from a properly sorted out cooling system in good shape should be negligible, so no doubt something's going on. I smell a head gasket (and/or worse), but there are several cheaper and easier things to look at.

I usually don't recommend "shotgun" replacement of parts (as opposed to diagnosis), but a radiator cap is trivially cheap and easy, and the thing has a lifetime of a few to several years so you probably need one anyway. (Amazing how much cooler an engine with an aging radiator cap starts to run when a good one is installed, even absent other physical problems with a cooling system. It's meant to run with a certain amount of pressure, and the radiator cap is one of the items tasked with holding that pressure.)

Nose around all the hoses, also, to be sure it isn't leaking out. And have a good look with a bright flashlight at the vicinity of the water pump when the engine is cold and again when hot (hands safely out of the way, of course). Soft-parts overhaul of a cooling system is not that difficult for the do-it -yourselfer nor very expensive, though perhaps where you live it isn't the season.

As others have advised, a pressure test is a good thing. I'm told that a smog tester can also be used to sniff for exhaust gases around the radiator cap.

Hoping it's something easy and cheap,

--Joe

Thanks for the information.

Dennis .