You should also note, that most systems are permanently closed. You will lose some water over time, and you do need to top off your loop from time to time.
Unless you have gas permeable tubing. You're not losing water, you're losing the air dissolved in the water. Once the air is gone from the water it becomes a better heat conductor and working fluid (and significantly heavier by volume. There's really quite a bit of gas dissolved in most water). In the pipe-fitting industry it's known as "dead" water. It smells chemically and bad. It's not good to drink, but it's great to heat your house (or cool your computer) with.
With a hot water heating system they put a valve from your water line directly into the system so there won't be any air getting in. This does pose a problem for a cooling loop though. You don't want to hook it up to your faucet.
Off the top of my head, an idea to solve this is maybe a ball valve on top of the system. when you want to bleed out the waste air, put a water source on top of the ball valve like a big tube, fill it with water 9may want to add that drop of biocide to it or boil it first or something), then open the tap. The air will bubble out, new (and sadly gas filled) water will trickle in. close the ball valve, tip the water out, and remove the hose. it takes a few months for the gas to leave the water, so you have to do it a few times.
A cool example of this effect is if you ever get ahold of old cold war era nuclear emergency rations they've got canned water in them. The stuff generally gets stuffed in some library basement ad sat for fifty years so all the gas dissolved out. if you shake the can, it rattles. The water doesn't want to reabsorb the gas, so it bangs around on the inside of the can. (and no you don't want to drink it)Edited by bombastinator - 11/12/13 at 8:59pm