The API water cartridge works - I used one for the first few months and it will give 0 TDS water. But, it runs quite slow, and can get expensive replacing cartridges, especially if you're in a hard water area. I eventually went to a 5 stage RO/DI and I run mine outside on a garden hose hookup.
The API unit actually runs faster than a RO unit and I agree it is a great choice. I used it for a few years before I bought an RO unit. If you aren't afraid of chemicals you can also recharge the resin yourself. Talking of convenience, recharging resin is NOT convenient. But it is a way of reducing water replacement to about 2 cents a gallon, if even.
The API unit is just DI resin with carbon prefilter. Technically, if you wanted you could just get a couple canisters used for purified water systems from Home Depot, put the carbon filter in the first and DI resin in the second and you have clean water. I'm not even sure if this is more expensive than replacing Brita cartridges, to be honest. You get DI resin from many online stores and some local ones as well.
Honestly, once you understand how the RO/DI system works, the whole thing is a joke. Water goes in, a few filters in between, water comes out.
My suggestion to you is this: Get an API Tap Water Filter
Tap Water Filter - API.
This will give you 0 TDS water as mentioned. Get a feel for it. The cartridges aren't too bad to replace and if you're lucky you won't need to replace them as often as Reefmack. Then do some research on RO/DI units. There is no rush. You can use the API unit for a few months, a year, or forever. It's just a DI unit but costs more to run than RO. But it's fool-proof and will relieve your anxiety about purified water until you understand it better.