Water Changes and Algae Filtration
Taking water out of your tank and replacing it with new saltwater is an excellent way to keep your tank cleaner and replenish elements: major, minor, and trace. You could keep almost any size tank without any dosing by doing small regular water changes, about 10% weekly. Some folks feel that doing water changes causes too much disruption to the tank inhabitants and they don't do any water changes. This can work if you are good about dosing all elements, but is probably not the best method for newer hobbyists. Regular water changes have been a reliable way of keeping a reef aquarium for decades and is still a solid practice I recommend.
Algae use nitrogen, such as nitrates, and phosphates as food. Incorporating macro algae such as Chaetomorpha or other colorful macros in your overall filtration scheme is very beneficial.
You can add a refugium and have macro algae in that or add some to your sump along with a light source, even a small incandescent clip-on light will work. The algae will use excess nitrates and phosphates as food and remove them from your tank, once you remove the algae as it grows then you also remove the nitrates and phosphates from the system.
Another way to use algae for filtration is to use an algae scrubber. These are sold commercially if you can find videos on YouTube on how to build your own. There are several ways of making one. You can make one out of screening and have water flow over the screen or have a reactor chamber and add macro algae and LED lights. However you do it, algae can be an excellent source for nutrient removal from your tank and help keep the tank more stable and clean.
I have kept many successful reef aquariums by using live rock and a live sand bed of about 1-2 inches deep. I use a good quality skimmer appropriate for the size tank operated by a pump that is more than adequate for that model skimmer. I recommend using high quality activated carbon on a continuous basis and making periodic water changes. The use of some macro algae as an aid to reducing nutrients can be very useful and can be incorporated in different ways. This should be all that is needed for most reef aquariums. Be aware that many reef keepers do not use a sand bed in their tank. Instead they rely on lots of water flow, up to 20 or more times the volume of the tank per hour, and a large skimmer to remove waste in the water. This bare bottom method does work well for tanks with predominantly smaller polyp stony corals such as Acropora. It may not be the best option for a mixed reef aquarium with soft corals and larger polyp stony corals. Small polyp stony corals can also thrive in systems with a sand bed and have for many years been kept by hobbyists with sand bed tanks also.
Sometimes you can find that you can over filter a tank, when I added ozone to one of the systems I noticed a sudden decline in the appearance of many of the corals, they looked less full and some just looked droopy and sickly. Upon stopping the use of ozone they perked back up within a few days. Some corals do not originate from such pristine environments and do not do as well when the tank is too “clean”. We can at times get too carried away with filtration and remove too much from the water. But again, ozone does work well in reef aquariums if used correctly, especially in small polyp stony coral tank where the desired result is very clean water.
Again, remember that there are many ways to reach the same end goal of a healthy and beautiful reef aquarium. There is no one correct way or wrong way and many folks have combined different methods and hybridized methods to reach the goal they desire. For a beginning hobbyist desiring an attractive mixed coral reef aquarium with a reasonable number of fish and invertebrates, the combination of live rock and a live sand bed with a good protein skimmer and periodic water changes is the best overall option of filtration.