If you depend on water exchanges to reduce or eliminate DOCs then your filtration isn't up to snuff IMO, but the vast majority roll that way I know. Using a ATS, my organic load stays very low without exchanges. Using Triton I get a exact view of 31 different essential elements. I monitor and replace those and all runs just fine. For most average hobbyists with smaller systems and no degree in chemistry
, water exchange is all you need, but for larger systems, replenishment also works. No crashes after 24 years, so I guess it works. But to make it work, you need a certified lab doing a broad range of calibrated tests, not home type testing. That is earlier where I almost went wrong in the whole thing, you have to test for a lot of different key elements.
I have a very heavily loaded system full of Stoney corals. I found out that:
Calcium was being depleted very, very rapidly along with Mg. Alkalinity was also being under dosed.
Lithium gets high without water exchanges.
Potassium gets depleted.
Iodine and Strontium as well as a few others gets used up fast.
A tiny, tiny bit of copper accumulates (Cuprasorb fixed that).
The Potassium was the big surprise. That's the result of running a ATS I think.
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