Yes i would say that he is at the point of the "viscous cycle". His tank sounds about the same as mine but my corals are probably larger and my demand may be higher. The only thing i dose is Kalkwasser and never have to add any 2 part except i will tweek the levels every month or so but never by all that much. I wouldnt worry about the ph at all mine sits around 8.0 and if i try to move it higher (such as in my battle with dinos) it just falls back within a couple of hours. Its a no win battle. Cal and alk are a funny thing as we all know, they work fine when they are all balanced but if you get 1 out of wack then it has the opposite effect on the other. This is partly why i prefer the Kalk route, as it is basically balanced to start with. I would check his mag too. I havnt had to clean a pump from calcification in a year and a half and some of the pumps have never been cleaned.

I have seen this before. Interested to see the mg levels.
Kevin

He doses magnesium weekly to maintain it at 1500

Not super low flow, it's a decent size sump but the mag 1800 is running at full open so the sump turn over rate is pretty high. The alk is being dosed automatically along with the kalk top off but calcium is added manually each day. I believe the full 400ml is added at all at once which could be an issue.

I believe he is using baking soda which he bakes in the oven first. I agree this is the likely the main issue, adding 10x the recommended dosage of this is going to cause a lot of precipitation which builds up on his equipment. The addition of Kalk is likely a factor as well which eats up all the CO2 in the tank, great for keeping algae growth down but not so good for dissolving all the additives.

Given that he is dosing Mg then I would agree with your last assessment. But also:

I think that is probably it right there, or at least that should be confirmed is the case how it's being added. At the best of times that's quite a sawtooth wave happening, that quick rise in levels is going to have an effect.

The Unbaked version (#2) of Randys recipe is the normal one to dose when using Kalk, although i dont know if this would cause any issues. The thing that seems so wierd to me is that he has to add it in such an un-balanced proportion. The 2 part recipies are meant to be dosed in equal portions but he is dosing double the alk as should be needed. Makes me think that something is way outa wack.

I would disagree based on personal experience. I have dumped far more than that in my sump with only a short time between the Cal and Alk being added and never have any precip problems. However if his levels are actually way higher than is indicated than it could be a problem.

You should try to test his water with your test kits and see the reading. My tank is about the same size as his, I also use Mag 1800 and I have not cleaned it for months. I did not clean my return pump for 2 year with my old one until the front plate got broken :-)

I dose around 80ml for both calc and alk with Randy recipe. But only have small- medium sps corals right now.

The only time I got precipitation was when I was doing twice the alk as the calcium, and then I was only adding 150ml of alk to 75 calcium and my pumps were fried in a matter of weeks.

This is first year chemistry:

It's an equilibrium reaction. Calcium carbonate isn't totally insoluble, it's just highly insoluble under normal conditions (like a Ksp of 4.0X10-9 or something). At any given moment in all of our tanks, a small amount of very soluble calcium and carbonate ions are reacting with each other and precipitating out. If things are in balance, the amount that precipitates out equals the amount of calcium carbonate that dissolves back in to the water, favouring the aqueous states of carbonate and calcium. It looks like this:
<-----------------------------------
1Ca(2+) + 1CO3(2-) <----> 1CaCO3

This is highly simplified standard temperature & pressure equation, and when things are normal, you need to worry about pH, carbonic acid/bicarbonate levels, magnesium levels, reaction of carbonate with other ions, uptake by corals, etc. to determine the exact nature of that relationship and how much calcium carbonate you'll produce. But in a balanced system, those other elements that affect that reaction will never lead to pump failures. We're talking about affecting the appearance of molar quantities of calcium carbonate so small you will never, ever notice it in a balanced system (and can barely even measure outside of a lab).

He's adding so much of each, and in such disproportionate ratios, that none of those other, more subtle elements are going to matter. When you provide a gross excess of either of those ions in a solution, you shift the arrow of that equilibrium reaction so that it looks like this:

------------------------------------->
1ca(2+) + 1CO3(2-) <----> 1CaCO3

all the other factors, pH, the relationship between carbonate and carbonic acid/bicarbonate, magnesium levels etc. - none of that matters if you dump so much of one of the primary ions in the above reaction in to the water that you shift the equilibrium way to the calcium carbonate side. Those things are still going on and are important in balanced systems, but the net result of what he's doing is exactly what he's seeing, ruined pumps on a weekly basis.

He's spending money on additives to plate his tank in calcium carbonate.

If I were him, I would stop dosing all additives until things have balanced themselves out. His levels will crash, which will be stressful on the system, but once they're down he can start bringing them back up in a balanced way again.