I set up a 38 litre frag tank in September 2025 mostly so I'd have somewhere to park coral cuttings, and I decided to log the whole cycle properly. Every number below came out of my own test kits at roughly the same hour each evening. Nothing has been tidied up to look neater than it was.
The reason I bothered is that almost every cycling guide I've read describes what should happen in tidy stages, and then people panic on day nineteen when their tank is doing something the guide didn't mention. Mine did several things the guide didn't mention. It was fine.
I used the prawn rather than bottled ammonia because I wanted a source that keeps releasing rather than one big spike, and because I couldn't be bothered driving out for ammonia. Both ways work perfectly well. The prawn is messier and it does smell for a few days, so if the tank lives in a bedroom, use the bottle.
Test kits used: a standard freshwater-style ammonia kit, a nitrite kit from the same brand, and a low-range nitrate kit. None of them are laboratory instruments. I care about the direction the numbers move, not the third decimal place.
| Day | Ammonia (ppm) | Nitrite (ppm) | Nitrate (ppm) | What I noticed |
|---|---|---|---|---|
| 1 | 0 | 0 | 0 | Water slightly cloudy from the sand. |
| 3 | 0.5 | 0 | 0 | Prawn has gone pale. Faint smell. |
| 6 | 2.0 | 0 | 0 | Bacterial bloom, water like weak milk. |
| 9 | 4.0 | 0.25 | 0 | Nitrite finally appears. Bloom clearing. |
| 12 | 4.0 | 2.0 | 5 | Ammonia stuck. This is where people give up. |
| 15 | 2.0 | 5.0+ | 10 | Nitrite off the top of the chart. |
| 18 | 0.5 | 5.0+ | 15 | Brown diatom film on the sand. |
| 21 | 0.25 | 5.0+ | 20 | Removed what was left of the prawn. |
| 25 | 0 | 2.0 | 25 | Nitrite finally coming down. |
| 29 | 0 | 0.25 | 30 | Diatoms peaking. Tank looks awful. |
| 33 | 0 | 0 | 40 | Dosed 2 ppm ammonia to test. Gone in 20 hours. |
| 36 | 0 | 0 | 40 | 50 per cent water change. |
| 38 | 0 | 0 | 15 | Three snails and a hermit in. |
| 44 | 0 | 0 | 10 | First frags: two zoanthid colonies. |
Day twelve, when ammonia sits still. It looks like nothing is happening. What's actually happening is that the first population of bacteria is still building and the prawn is still releasing. If you do a big water change here to "help", you've just diluted the food supply and set yourself back a week. Leave it alone.
Day fifteen to twenty-five, when nitrite reads off the chart. Nitrite kits max out around 5 ppm and mine sat pegged there for ten days. There is no way to tell 6 from 40 with a hobby kit, so you just wait. This stage is properly slow and properly boring, and I reckon it accounts for more abandoned cycles on its own than everything else put together.
Day twenty-nine, when the tank turns brown. Diatoms. Ugly rust-coloured dust over the sand and glass. They're eating the silicate that came in with the dry rock and the tap water, and they run out of food on their own. Mine cleared in about twelve days with no intervention beyond a turkey baster.
If I had one sentence for a beginner it would be this: the cycle finishes when the tank can eat 2 ppm of ammonia in under 24 hours and show no nitrite. Not when the calendar says six weeks, and not when the water looks nice.
I've cycled tanks with and without it. The bottle did not, as far as I can measure, make my cycle shorter — six weeks either way. What it seemed to do was make the ammonia peak a bit lower and less jagged. That might be worth something to you or it might not. I'd never rely on the "instant cycle, add fish today" claim on the label, and I'd never do it with a fish in the tank.
The frag tank has been running eleven months now. Nitrate sits between 5 and 15 with a small weekly water change, and I feed it deliberately because a frag tank with zero nutrients grows nothing. If you want the ugly details of what my dosing looks like once a tank is mature, that's the alkalinity post. If you're wondering what to actually put in the thing, I keep running notes on the livestock page.
And if your cycle is doing something that is not on the list above, send it over with your numbers through the note box on the about page. Nine times in ten it is nitrite pegged at the top of the chart and the answer is patience, but I reckon I learn more from that tenth one than from anything I have read.
Glass Reef — a hobby diary typed up in a spare room in Brisbane, Queensland. Nobody sponsors it, there are no affiliate links anywhere in it, and there is nothing on it for sale.
Copyright © 2019–2026 Dale Prentice. Back to the front page.
Last fiddled with on 3 February 2026, when I corrected the day 33 ammonia figure that I had typed as 0.2 ppm.