When to use chemi clean?

[EnterName]

So there can be nutrients even when my hannah checkers read 0?

Yes and no :D

Of course there is a detection limit of your HannaCheckers, but organisms can't drain nutrients indefinitely. The resulting starvation will release nutrients back into the water and each organism's uptake mechanism might put a limit on how low they can drain said nutrients.

The question is: How low can those levels drop before your corals actually start to starve?

Nitrogen/Ammonia/Nitrite/Nitrate:
If your nitrate test shows 0.00ppm that doesn't mean the tank does not provide enough nitrogen. It's very likely that all available ammonia gets consumed before it is oxidized to nitrite or nitrate and therefore your test simply doesn't detect anything. Corals and algae actually prefer ammonia as they can get the nitrogen they need directly. Nitrite and nitrate need to be reduced first with enzymes which costs precious resources (e.g. trace elements), energy, and time. You can have a reef tank with no measurable nitrate but still enough ammonia to allow for fast growth. In fact, in actual nutrient-limited systems you will see that dosing ammonia does not directly increase nitrate. It will look like nothing happens, and only if you add more than the living organisms can consume within a certain amount of time your nitrate levels will rise. (This is a bit simplified as nitrifying bacteria will always oxidize ammonia when they get it, but it describes what you would observe with your tests. No changes in nitrate until you hit a certain threshold.)

Phosphorus/Phosphate:
For phosphate it is a bit more complicated. There is an ongoing equilibrium reaction that binds phosphate to rocks and substrate and re-dissolves it. If your readings drop too low, the equilibrium will shift towards re-dissolving phosphates and your rocks/substrate will effectively act as a buffer against phosphate depletion for some time (that's why sometimes water changes don't appear to drop phosphate levels much). If dissolved phosphate is kept too low for too long this buffer won't supply phosphate fast enough (or at all) and the tank hits an actual phosphate limitation. When trying to increase phosphates in such a system it will take more than you would expect as you have to saturate the rocks and substrate again before the equilibrium allows for enough dissolved phosphates.

I know it's a lot and still a bit simplified... I hope this helps :)

 

[CHSUB]

Corals usually only meet a fraction of their nutritional needs through digestion (heterotrophic feeding supplies 15-35% of the daily metabolic requirements and 0-66% of the fixed carbon into coral skeleton). If non-detectable nutrient levels are working in a tank, it's very likely that (for example) ammonia is available but gets consumed before it is being oxidized to nitrite and nitrate making it "undetectable" with nitrate tests. In an actually nutrient-limited system (rare) feeding might not be enough until the excessive food is broken down and provides the required carbon, nitrogen, and phosphorus, which is well known to cause coral bleaching when deficient.


We can detect nutrients at very low levels with ICP, but that doesn't mean organisms like corals are still capable of consuming. Their uptake mechanisms have to allow for the assimilation on a biochemical level. Bacteria will probably be able to deal with lower nutrient levels than even dinoflagellates (and therefore zooxanthellae), but I would have to take a closer look especially when considering cyanobacteria.

In other words: It very well appears to be possible to have scenarios in which bacteria can grow but corals would starve.

I fully agree with you that there are most likely more than enough nutrients in OP's tank to have the readings drop quite far (even undetectable) without any issues for quite a while, but I would be careful to not oversimplify these things.

I agree, except here, “but I would be careful to not oversimplify these things.” Hobbyists that are seeking advice on algae are not benefiting from advanced discussion, imo. New hobbyists are reading and advising others about algae and nutrients without an understanding of basic biology. An aquarium that is growing algae is not suffering from lack of nutrients. Some are reading scientific literature about dinoflagellates surviving in low nutrient conditions by consuming Cyanobacteria that fixes n2 as a survival mechanism themselves and applying that to reef aquaria.

Advice on algae in the reef aquarium should be, “see lots of algae=dirty aquarium that needs cleaning”.

 

[EnterName]

Advice on algae in the reef aquarium should be, “see lots of algae=dirty aquarium that needs cleaning”.
[...]
An aquarium that is growing algae is not suffering from lack of nutrients.

I agree, but there are exceptions and I can't fully rule out the possibility that there are cyanobacteria that do fine in systems that would be considered nutrient-deficient for other organisms. Cleaning up is still the go-to approach at first. To quote myself (post #7 in this very thread):

That's why bottoming out nutrients during a cyano outbreak might not be a bad thing.

Hobbyists that are seeking advice on algae are not benefiting from advanced discussion, imo.

That's the good thing about forums like R2R, you can pick your poison yourself :D. You will find both approaches here, and I think this is very beneficial.

Some are reading scientific literature about dinoflagellates surviving in low nutrient conditions by consuming Cyanobacteria that fixes n2 as a survival mechanism themselves and applying that to reef aquaria.

Not every study can be applied to reef aquria, that's completely fair, but the previously referenced studies (like phosphate deficiencies causing coral bleaching) had to be tested in tanks/culture vessels to control the available nutrient levels and can be easily reproduced in tanks when not providing a phosphate source.