So, I've been reading some other forums . There seems to be a consensus from some experts and others more knowing than myself, that one should always aerate and buffer any RO/DI (and I'm assuming also distilled) water used for reefing. This apparently applies to both top-off water and water used to make saltwater mixes. The idea is that because RO/DI water is so depleted of any ions and buffering capacity, and because it is very low in pH, with high carbonic acid and nitric acid levels, if you don't aerate and buffer it, it taxes the buffering capacity of the tank (or salt mix) as the carbonic and nitric acids "eat up" the alkalinity, leading to low system alkalinity.

I understand the need to aerate to drive off CO2 and so always aerate my water for making up new saltwater. Starting today, I'll also drop an airstone into my top-off water bucket too. That problem is solved. My real question has to do with adding the buffer to top-off water. Assuming that basically only pure water evaporates from my tank while the ions and other disolved components that make up saltwater stay behind, if I top off with pure distilled water I'm replacing just the water that evaporated right? Now by adding a buffer, which is usually sodium bicarbonate to the top-off, I'm adding more sodium ions to the system (which never evaporated to begin with right?). So, over the long run am I not adding too much sodium, causing it to rise in relation to the other ions that make up saltwater? This would shift the balance of ions in my tank away from natural seawater over the long run as my Na would rise, particularly in comparison to Cl based on my understanding of the chemistry involved. I know people have a similar problem with long term use of calcium-chloride to boost calcium levels. They end up with unnaturally high levels of chloride versus sodium and it messes up their params. Wouldn't the reverse happen adding buffer to top-off water? (PS I'm not worried about the carbonate part of the equation because I understand that part of the sodium bicarbonate is easily used up by my livestock, however, the Na ions are not.)

I love this hobby but the more into it I get, the more I think of this stuff. And, I hate doing things just because somebody says so, without understanding it for myself.

Thanks.

If you're doing regular water changes there is no need to worry about chloride levels from dosing calcium or buffering your RO/DI water. any "ions" depleted will be replaced with regular water changes. you may run into a problem if you went many months without water changes in a smaller system, but no worries if you're properly maintaining the system. plus once the tank is established you'll be adding alkalinity buffer for the whole tank to the top-off water anyway.

no reason to buffer top-off water... all the goodies are still in the tank. You just had water evaporate. The reason ro/di top-off water has "low" pH and "low" buffering capacity siis because it's pure water. When you add it to the tank, it mixes with all the goodies and all is well in the universe.

This concern was why the original dosing regimen was calcium hydroxide. The hydroxide reacted with carbon dioxide to produce bicarbonate so no wasted ions where present to get out of ionic balance.
Limewater is still good because of that reason (not for other reasons)
Since then, it's been found that with regular water changes, using calcium chloride and sodium carbonate doesn't really cause a problem of ionic imbalance.
The reason to aerate the water (and it can be done in several ways) is due to the intial formation of ammonia in the freshly mixed up saltwater. Aerating the water displaces the ammonia from the solution and also the cabon dioxide reacts with the ammonia to keep it from consuming alkalinity.