Coral keeping information.

Firstly you need to establish the best biological system you can to run you aquariums nitrogen cycle. This is more important then anything else when keeping corals.
Water conditions needed for corals on average.
Temperature.
The average temperature for your reef aquariums water should not exceed 27c and equally should not fall below 22c;the best constant temperature for all corals is 24c-25c.
This is because available oxygen is at its greatest with in this range and the many nutrients and other coral aggravating impurities will take on greater polluting potential when your aquariums temperature exceeds 26c.
The coral creature its self has no real issue with temps reaching 29c,it is the corals life giving algae with in its cells that will respond badly to these changes and begin to poison the coral creature from with in, from this it is expelled and bleaching occurs.
To help corals that have had a brown out, their symbiotic algae turn brown!
Many say that Zoox/zoozanthellae build up in numbers due to lowlight or high nutrients.
For a quick assistance you could dose a carbon source and nitrogen or manufacture these naturally in your waters your self, then I would ad carotene and iodine with halogens as a good beginning!
The brown colouration is actually resulting from the dominance of the xanthophyll pigment fucoxanthin, this pain in the butt masks the other pigments that enable the corals to show other colours and photosynthesis correctly. These xanthophyll pigments are Chlorophyll A and C ,B doesn’t exist, then beta-carotene and the usual xanthophylls.

Helicostoma nonatum or brown jelly coral disease.
All large extended polyp corals like elegant, hammer, goniopora to name just a few and some acropora are susceptible to these attacks!
These are after the fact to the destruction of coral tissue from bacterial toxins and enzymes from species like vibrio sp and vulnificus bacteria that would be classed as a virus.
This is able to happen due to unsuitable conditions or in the vicinity of other more aggressive corals species causing that coral harm and will bring on wide spread tissue stress/sickness weakening its immune system.
Once the ciliated protist gets into the corals flesh after bacterial issue opens it to attack to feed on the symbiotic algae with in, there is not much time to save it, if it can be saved!

First and for most the reason this came about is due to low water quality!
Also a very much contributing factor is far to slow of water flow/current in your aquarium!
These issues are always from things like depressed water quality, low oxygen and slow and or weak one-way flow/current!
If it gets a hold of your coral, dip it in an RO and iodine mix twice a day for 5 to 8 days and always use activated carbon if this even slightly appears again.
Coral born toxins that are used to attack their neighbours that they sense via what you could call a sense of smell, will harm other corals, also any nitrates sicken corals as do orthophosphate and phosphorus.
Water flow should always be between 15 to 25 times your aquariums water volume.
High temps kill corals by reducing water quality!

Bleaching.  
There are many reasons for corals to expel the varied available algae types they up-take that make up the clades of algae that the corals have selected from the waters, you provide for them.
The word “algae” provide a strong hint as to why this can be a very delicate symbiotic relation ship.
Algae are inherently nutrient importers and not all nutrients are conducive to being converted into beneficial foods and elements for coral health and growth.
Corals expel algae primarily because of heat and or excess useless nutrients that may be present and will be imported, that is what algae does!
1.Algae may be manufacturing far too much co2.
2.The algae may begin to fail in its photosynthesizing of nutrients into glucose.
3.When the algae’s age dictates the weakening of its potential in extracting supplements needed for the corals skeletal construction.
4.When the algae has become less efficient at reproducing new algae with the corals cells.  
Nutrients that inhibit coral health, to beneficial nutrients for coral health.  
There is only one naturally forming nutrient in your aquarium that is of a detriment to coral health. Before that, there are two grades of nutrients to be aware of, inorganic and organic. Organic nutrients are of no real concern in a matured reef aquarium for your corals.
Inorganic types are of a serious concern when they are in abundance and especially if there are any inorganic nitrates lingering in your waters! Nitrates are found in abundance when the anaerobic zones in your waters are not large or extensive enough to separate the atoms that make up nitrate. Nitrogen is left over when anaerobe-microbes use the oxygen atoms leaving only pure elemental nitrogen gas, if this is not working correctly, you are now faced with another adverse element for coral health.
Nitrate inhibits the corals use of calcium carbonate, slowing the corals growth considerably and if these elevated levels continue, the corals will die!
For coral health you need low levels of what tests as phosphate, being orthophosphate and phosphorus and zero results in when testing for any of the components of the nitrogen cycle!
Flow, for coral health.
This is very important for the cleaning of sps, making added foods quickly available to the corals, moving oxygenated waters across the corals and keeping water consistent in all areas of the aquarium.
The flow in your aquarium should be moved at no less then 15 times per hour or greater, then the volume of water in your the your aquarium.
The flow should be making ripples on the surface of your waters to achieve the valuable “broken surface tension” or PH may fail!
Rock structures for corals.
Keep your live rock less in diameter, meaning thinner, like branching acropora live rock. If possible go with base rock as it has less excreting sponges and other toxic life to corals and no hitchhikers that may attack your corals.
Do not have a substrate of any kind in the display aquarium, just small sections of base rock covering the floor that from above, has lots of gaps, but from the front and sides, you can’t see the gaps. This will look fine and house microbial life and more.
Food for corals.
Corals exist at their best with live food as is phytoplankton and rotifers. When these are added, if they not eatern, they diminish in size with starvation and by the time they die, there is very little body to decompose and increase nutrients.
Corals consume plank tonic sized fresh waste particles and plank tonic sized creatures, in the same way as an anemones!
The corals expel the waste from these foods, which is imported by the corals algae with in and processed to glucose and this is passed to the coral creatures as is valuable elements from uptake by the algae.
The algae will import available nutrients in the waters as well, so these should be at a minimum for coral health or this action will poison the coral. There has to be small traces of nutrients or the corals will die!
Salinity.
Best is 1.024 to 1.025.
Corals are very basic inverts and need constant salinity or they stress and will fall prey to bacterial out breaks.
Bubbles and their affects on corals.
Where most of your corals are collected from or where they have been fraged,there is at worst, a slight bubble content,if any.
Micro or standard sized bubbles will aggravate corals and over along period of time, may kill them!
Lighting for corals.
LEDs are the best lighting for all photosynthetic life and these should consist of at the least,3 watt 80 degree diodes mostly of cool white and blue diodes, with a small amount of orange and red diodes with a slightly larger optic spread of their light.
SPS are best closer to the light,lps further down with toxic elegant types like hammers as such, acans around the base of your aquarium, then gorgonian and tubastrea in low light areas with strong flow.
Acans placed up high, will display very strong colours,but if waters are not of a high quality, this area may eventually kill them over time.
General coral health.
When purchasing branching acropora, make sure each piece has an acro crab on board; they help keep the acro clean and protect it as well.
The corals with smaller polyps and extensions are more prone to be attacked by other corals.
Never put large polyp species with in 30 to 50 mills of small polyp species.
The toxins expelled by large or extended polyps species will slowly kill small polyp species in some reef aquariums if external or internal algae is not abundant or you do not have a chemical filter in place to reduce naturally formed toxins!

DIY supplements for corals.
What follows is how to make most of what your reef aquariums corals require to exist easily and attain slow or fast growth.
Calcium.
Dissolve 250 grams Damprid from bunnings in roughly 2 liters of RO water.
Just be aware the mix gets a bit hot and with its reaction when the two are combined.
Calcium citrate
Any calcium coral, seashells, limestone, or dolomite is fine. Then combine with citric acid, lime or lemon will do nicely.
Alkalinity
Buy some pool alkalinity and PH up from Bunning's and mix 350 grams once it is cooled to 2 liters of RO water and mix for quite a while.
Magnesium.
Grab 2 cups of Epsom salts and add near double that of magnesium chloride as hexahydrate in 4 litres of RO water and mix. This solution is added much less frequently than the other two parts. But can be added in a large amount at once.
Iodide is needed regularly for the algaes pigments especially for the corals algae.
Iodine,do not add raw iodine use it with halogens preferably and it is introduced natural with any foods you may use and the resulting iodide converts to it with in reason any way.