What makes a reef aquarium crash and how to stop it from crashing.

Virtually all aquariums crashes for the same reasons, quite often it is toxins from the live rock life forms or introduced toxins, but nearly always it is from PH disruption!

In brief!
When your tank crashes the “cycles” bacteria suffers near no ill few affects by what killed your marine life!
Much of what you are advised on are based on your tank functioning perfectly or, under the control of dosing pumps, endless testing or vigorously adding and testing your self, a very large system and much more and if those are the case, you could get away with 25c or 27c like the incredibly immense ocean gets away with, that is of course if you have a high end set up or an incredibly massive expense of water for your hobby?
If not you will be advised to spend more and that’s cool if you are happy with spending more.
Any at all CO2 and ammonia both take out the capacity for KH to be affective of which keeps PH stable, you know alkalinity, the life sustaining acid binding carbonates business that goes on in your aquarium that you can’t see!
It’s up to each reefer/fish only hobbyists of course, but the simplest ways to not let the PH collapse that causes most tank crashes is to add a little magnesium each day as with a little of the bunnings pool product called KH and PH up! Its virtually impossible to over dose magnesium but very easy to under dose!
As an example only my massive algae section in play working correctly still sucks my water dry of carbonates, my KH and PH collapses after 2 to 3 days if I do not dose with varied mags and that pool product!
Most reefers aquarium bacteria, “the cycle” that you need to keep your tank life alive “produces acids!!!”
PH is the gap between an acid and a base and if you add in more acids with out additional carbonates and it closes up, PH drops, oxygen is toxic, dead tank life!!!
What happens is that stuffs with the balancing Carbonates that keep your KH and subsequently PH stable and of course anything that is symbiotic with algae, so obviously all corals and all actual algae take out carbonates as well and once lights are out, all these use available oxygen in your water once lights are out and photosynthesis is no longer functioning and like everything else, their waste production is now CO2 and not oxygen.
From day one !!!!if you want a good tank, lightly dose!!!!!!! And keep dosing and the same day put in Phos control, these two from day one!!! and never let them fail or expire!

These are what affect your PH-
Low oxygen from lack of gas exchange, temperature, ammonia, etc, then you get disruptions.
Excess CO2, from when the lights go out and corals algae and other standard algae with in your water emit CO2 or from algae and animals in abundance once they die, or the house is closed up and maybe the heater is on and this will inspire excess CO2, more disruption.
Any or all of these will kill your aquariums life forms, either quickly or via each individual fishes stress response causing cortisol damage, which equates to either neurological damage or the weakening of the fishes immune system resulting in parasitic attacks.

To avoid tank crashes-
Tank crashes from excessive water temperature and why it should at 24c.
The issue with temps in saltwater is quite vast and its not just about saying solubility of oxygen with what ever temps, its about what is affected by rising temps disallowing oxygen solubility and much more.
As temps rise above 20c, solids become more soluble and this can get quite ugly for fishes gills, bacteria function much faster on all levels and not all are good, co2 is much more common place, fishes iron pumps and kidneys are working harder, all is metabolised much quicker, your fish process what they eat much quicker adding a lot of extra ammonia and then need more feeds and subsequently pollute your water even more.
Do not rely on skimmers to do much in the way of gas exchange, like oxygen uptake, ammonia released into the air, nitrogen released into the air and CO2 released into the air as well.Use cascaded waters to achieve these.
Make sure there is sufficient current breaking the waters surface to help with gas exchange by alleviating extremely detrimental surface tension issues.
Any film or oily coating on the waters surface will eventually crash an aquarium by preventing the breaking of surface tension.
The aquariums waters have to be skimmed to avoid surface tension issues, that means the water over flows from your aquarium dragging the highly polluted surface waters out to be processed. All pollutants are concentrated at the surface!
To avoid the algae in the aquarium, both symbiotic with in your corals and all other algae from causing PH disruption from CO2 emissions, have an algae area externally with the lights on over it when the lights are off in your aquarium.
The algae with the lights on will import the disruptive CO2 emitted by any other algae types.
Do not use a deep sand bed in your aquarium; the resulting sulphur from hydrogen sulphide has the potential to cause a tank crash.
Do not over feed beyond the capacity of your biological media.
A bio system has to be matured to handle waste properly!
Do not suddenly change anything like KH, Temperature, salinity or current direction where you have your bio filtering when the flow is usually in one direction.
If you move live rock, add PH stabilisers.
Do not dose anything in a large amount.
Do not dose with iodine, just use iodide instead.
Clean your skimmers impellor each month and replace it if it is worn at all!
Do not change lighting with out adding something to stabilise your PH.
Do not do a large water change with out adding something to stabilise your PH.
Your algae will turn sexual if it is deficient in iodine, which comes from iodide and fish food,also iron and all the usual coral dosing elements.
Have lighting of standard blue and cool white diodes over the algae, resistance lighting will fade and this will kill the algae.
Never let your algae importers have to with stand any temps beyond 26c,it will usually, in time kill it.