so how do we decide which one is for our system?
Many factors. Let this thread run for a day or two and come look and see if there aren't several different answers.
What made the decision for me was that my evaporation rate wasn't stable enough to put kalk in the ATO and get a stable dose. So after trying that for a few months, I went to two part.
Kalk is easier to do than anything else, and it's harder to mess it up. No matter how much kalk you put in your ATO, only so much will dissolve and most people dose it saturated like that. You're also dealing with larger volumes and lower concentrations, so if you mess up your dose, the consequences aren't as disastrous.
The other big concern with kalk is that it has a very high pH when mixed with water. So you have to make sure it goes in really slowly or it will spike your pH. People who have low pH will often take advantage of this to get a pH boost at night when pH is the lowest. But if your tank runs at a high pH normally, then you might find that each addition of kalk causes a dangerous spike.
The two part solutions take a little more finesse to dial in. You have to learn to watch trends and understand that the immediate effect of changing a dose may be very different from the long term effect. That's the main disadvantage of two part. It's really easy to do wrong and end up using very very large volumes of additives to maintain your levels because most of what you add just precipitates onto your sand. So instead you make slow changes and watch them work out over a couple of weeks at a time.
With two part you are using highly concentrated solutions and very small volumes. This means that messing up your dose can make a bigger difference than with kalk. But it also means you will have a more precisely set level of dosing.
You also have a choice in two part recipes. Recipe 1 is made with sodium carbonate and will have a pH boosting effect. It also doses at double the rate, so you use half as much solution. This pH boost can be dramatic, but since you get to control the dose by the milliliter, you can spread it out over the day. The second recipe uses sodium bicarbonate and has very little effect on pH at all.
Another big difference is that kalk can't really be used to raise the levels in your tank. Only to maintain them. You would still need an additive to use when levels got low. This is because you are limited in how much kalk you can add at once not only by the pH issue, but also just by the sheer volume. You're eventually going to hit a point where you are adding it faster than your evaporation rate and you are going to crash your salinity if not flood the tank.
With two part, you can simply make an additional dose of a few extra mL to boost a level that's low. If your salt mix has low calcium for instance, you could add an extra 50mL of calcium part whenever you do a water change. That's not something you do with kalk.
If you have a relatively stable evaporation rate, stable low to normal pH, and a good ATO then I would say start with kalk and see if it works out. I guess I'm just lazy, but if the lazy man way gets it done then problem solved.
If on the other hand you want a little tighter control over the levels, then go for the two part on a dosing pump system.
And we haven't mentioned calcium reactors yet.
