Why my alkalinity kept sliding, and the boring fix that stopped it

Posted 4 June 2026 · filed under water chemistry · about a twelve minute read

Between about October 2025 and May 2026 my alkalinity fell roughly 0.9 dKH a week and I could not work out why. I want to write down the whole embarrassing investigation, because the answer was three separate small things stacked on top of each other and not one clever thing, which I think is how most of these go.

The symptom

I test alkalinity every Sunday morning. From spring onward the pattern was always the same: I'd correct to 8.4 dKH on Sunday, and the following Sunday I'd read 7.5. Then I'd bump the doser up, and within ten days the tips of my one decent staghorn colony would go chalky white and I'd have to back everything off again. Round and round for the better part of seven months.

What I believed was happening: coral demand had grown and my doser wasn't keeping up. What was actually happening: my doser was delivering less than it claimed, my test result depended on what time of day I took it, and I was reading the burn as an alkalinity problem when it was a rate-of-change problem.

Thing one: the pump was lying to me

A peristaltic dosing pump squeezes a flexible tube against a roller. That tube is a consumable. Mine had been in service since the tank was set up, which is a genuinely stupid length of time, and it had gone soft enough that each rotation moved noticeably less liquid than it did when new.

I finally measured it properly, which takes ten minutes and which I should have done a year earlier:

1. Pull the output line out of the sump and into a measuring cylinder.
2. Tell the pump to dose 100 millilitres.
3. Look at what actually landed in the cylinder.

I now replace dosing tube every six months whether it looks tired or not, and I re-run the cylinder test on the same day. It sits on the same calendar reminder as the smoke alarm batteries, and if it did not it would never get done at all.

Thing two: I was testing at the wrong time

Alkalinity is consumed all day and topped up by the doser overnight, so the reading you get depends on where in that cycle you sample. I had been testing "Sunday morning, whenever I got up", which in practice meant anywhere between 6 am and 11 am depending on whether I'd been out on the Saturday night.

Over four weeks I deliberately tested the same tank at four different hours:

Same tank, same kit, same week, different hours. Four separate Sundays.

Time of day	Reading (dKH)	Where the doser is in its cycle
06:15	8.5	Just after the overnight doses finish
10:30	8.3	Lights have been on two hours
16:00	7.9	Peak consumption
21:00	7.7	Lights out, doser about to start

That's a 0.8 dKH spread inside one day, from a tank that is perfectly stable. Which means a good half of the "drift" I'd been panicking about across seven months was me comparing a 6 am reading against a 10 am reading and inventing a trend.

Pick one hour. Test at that hour. A wrong number taken consistently is more useful than a right number taken whenever.

I test at 7 pm on Sundays now, because that's the time I'm reliably home and not in a rush.

Thing three: the burn wasn't from the level, it was from the jump

Every time I got a low reading I'd correct it in one go — a manual dose to bring 7.5 back up to 8.4 in an afternoon. That is a 0.9 dKH step change in a couple of hours, and small polyp stony corals hate that far more than they hate sitting at 7.5.

The white tips always showed up two or three days after one of my "fixes", never during a slow slide. Once I connected those two facts the pattern was obvious and I felt like an idiot.

The rule I work to now, which I pinched off a bloke at the local aquarium society and have never once regretted:

• Never move alkalinity by more than 0.3 dKH in a single day, and preferably not more than 0.2.
• If you're 1.0 low, that's a five day correction, not an afternoon.
• A stable 7.4 beats a bouncing 8.5 every single time. Corals adapt to a level. They don't adapt to a graph that looks like a saw.

Where things sit now

Current parameters, 220 litre mixed reef, measured 31 May 2026.

Parameter	Value	Notes
Alkalinity	8.2 dKH	Varies 8.0–8.4 across a week now
Calcium	425 ppm	Tested monthly, barely moves
Magnesium	1310 ppm	Topped up with the monthly water change
Nitrate	8 ppm	Deliberately not zero
Phosphate	0.06 ppm	Was 0.00 for a year and the corals sulked
Salinity	1.025	Auto top-off keeps this flat, see the top-off post
Temperature	25.8 C	Summer is the hard part in this city

The staghorn has grown about four centimetres since February and hasn't burnt once. That's the whole result. Seven months of investigation to arrive at "replace a consumable, test at a consistent hour, and stop making big corrections", which is not a thrilling conclusion but is apparently one I needed to learn by doing.

What I still don't know

I don't have a good explanation for why my consumption is about fifteen per cent higher in February than in July, given that the tank is indoors and temperature-controlled to within half a degree. My best guess is light — the room has a west-facing window and the summer sun spills across the tank for an hour in the late afternoon even with the blind down. I've never measured it properly. If you have got a better theory than that I would genuinely like to hear it, and the ways to reach me are on the about page.

If you're earlier in the process than this, the cycling post is the more useful place to start, and the livestock notes say which of my corals are actually fussy about any of this. Honestly, most of them aren't.