Phosphate and silicate
Phosphate and Silicate in the aquarium are responsible for most nuisance algae outbreaks. Testing for phosphate is easy and there are many test kits available, but test kits accurate enough to test silicate are currently not within the reach of hobbyists.
Luckily though, most phosphate removers also remove silicate, so it's not too difficult to keep these levels low.
I recommend either the D&D or Salifert Phosphate test kit is used weekly, and results kept. By doing this you will learn how long your phosphate remover lasts, which means you won't either a) run the tank with high phosphate levels, or b) throw away media that still has life left in it.
Most test kits are not accurate enough to test for the low levels of phosphate, in a reef aquarium you're looking for < 0.03ppm PO4.
On the salifert scale this is the first coloured comparison square next to zero, which is clear. To make it easier to use the Salifert kit (and avoid spending money more expensive kits), do the following:
In addition to the usual instructions in the Salifert PO4 test kit; when comparing the colour to the chart, put another vial of 10ml of tank water next to the one with the chemicals inside it, and the difference in colour will be very much easier to see!
Calcium, Alkalinity & Magnesium
Hard Corals depend heavily on Calcium and Carbonate to produce skeletal mass. Levels of these three elements must be kept above a certain level in order for coral to thrive, and maintaining these levels is a very important part of keeping your aquarium stable.Luckily there's a huge range of additives available, and the test kits are simple to use and accurate.
Calcium is combined with Carbonate by coral to produce it's skeleton for growth. The following levels should be maintained:
Magnesium 1250+ ppm
Alkalinity 8.00 KH or 2.85 meq/l (minimum)
In soft coral-only tanks it's possible to keep lower levels of these elements without many problems, and many people with only soft corals usually don't own the test kits anyway.
However, Alkalinity is important because it buffers the pH level, which is very important to all of your livestock. A pH level measuring 8.2 - 8.4 in the evening before lights out is perfect. It's best if it remains high all day long, but most aquarists can't achieve this, and have a ph 'swing', usually meaning their pH can be as low as 7.8 in the morning.
Keeping the Alkalinity high will help somewhat to avoid this.
A high pH and alkalinity will also reduce the chances of outbreaks of slime algae on your coral sand. This is simply because coral sand contains phosphate/silicate that the algae feeds off, and the sand is dissolved in the aquarium much faster at lower pH levels.
What about Magnesium? Maintaining a proper magnesium level gives stability to maintaining Calcium and Alkalinity levels. A low magnesium level can harm certain organisms, and usually makes maintaining the other two elements very unpredictable. Again low Magnesium can encourage hair
Solving algae problems
General rules for combating algae problems:
Phosphate level is low <0.03.
Nitrate level is low <5ppm
Water entering the aquarium has a very low TDS.
Alkalinity and pH are suitably high.
If all those are correct then you can consider two options:
1) Use livestock to phsyically eat algae.
2) Use chemicals to kill the algae.
The benefit of option 1 is simple - it's natural. The downside many people forget it that the livestock introduced may simply not be interested in eating the algae, and you often have to get the right species in the first place. In addition, you'll need to bear in mind that once the algae has gone, will your new critters starve?
Option 2 is a very common solution for Red Slime algae. As this algae is actually a bacteria, there are many products that simply starve the bacteria and get rid of it quite easily. Before purchasing these I would suggest you make sure your parameters are all in check, or the red slime will be back soon.
There are a few products designed to kill green hair algae and they do work but not as well as you'd like. Firstly they need to be squirted directly onto the algae. If you hit a coral with these chemicals it will get burnt. I don't recommend these products, their use is simply impractical.
Parasites, Bacteria, Viruses, Worms