[Delphinus]

True.... although ... it's still better to add it slowly than all at once. It takes time to mix in and become homogenous.

Now that I think about it some more, that was something I used to do when I had calcium buildup in the past, I'd just do Ca in the morning and Alk at night and just dumped it into a fast moving part of the sump or the overflow itself. It still takes a little while to dissipate into the overall tank volume and in that time what's happening? This is going back like 10 years or more so memory is faded a little, but I did used to dose 2-part, then I thought that was for the birds, went to a Calcium reactor and also thought it was remarkable that the Ca reactor, although maintaining less ideal values than the dosing did, the growth in the tank was better (and the calcium buildup was gone). Fast forward a few years and the pendulum swings the other way, I conclude that CO2 cylinders are for FW tanks (instead of birds this time) and I go back to dosing but this time using timers and slow dosers, adding slowly and spread out over the day (while still alternating Ca/Alk) and yeah it's like the best of both worlds. Nicer params, good growth and no ca buildup in impellers.

In your friends case it's probably a combination of factors that each on their own wouldn't be enough to cause it but in concert together they are.. That's my guess anyhow. Should be interesting to see, 9 weeks should be plenty of time to get it sorted and settled, methinks anyhow..