Being the only filter, the problem is when you clean it you have no filter for a few days. So having two units fixes this.
The basic guidelines for algae scrubbers is based on how much you feed each day: 1 cube a day, 2 cubes a day, etc. However these are just starting points; a lot of your tank filtering is based on your rocks, so their condition plays a part too in what model(s) scrubbers to get or make, as well as what type of feeding you are doing, and what other filters you will be using.
REEF:
1) If you are building a reef tank which is new, meaning that the rocks are coming from the ocean or from a low-nutrient tank, and if you will just be feeding the fish sparingly, and if you DO want to have other filters and water changes, then you can just use the cube-feeding recommended sizes of the scrubbers.
2) If you are building a reef tank which is new as in #1 above, but you DON'T want any other filters or water changes, then double the recommended scrubbing amount in #1. This will supply the corals and small fish with the most amounts of food particles. You don't need to start the tank with all the scrubbers; one is fine for a few months. Add the others later.
3) If you are building a reef tank which is new as in #1 or #2 above, but the rocks are coming from a nutrient-problem tank which had measurable phosphate or hair algae problems, then the rocks will be soaked with phosphate and this will supply more phosphate to your new tanks than the feeding will. So double the recommend scrubbing amount. And if it is a new reef tank with problem rocks AND you don't want other filters or water changes, you would need four times the scrubbing in order to handle the problem rocks and the other filters.
4) If you are adding a scrubber to an existing reef tank, and the tank has no measurable phosphate and no nuisance algae, and if you have other filters and water changes and you DO want to keep them, then you can just use the cube-feeding sizes of the scrubbers.
5) If you are adding a scrubber to an existing reef tank as in #4 above but you DON'T want to continue using the other filters or water changes, then double the scrubber amount recommend in #4.
6) If you are adding a scrubber to an existing reef tank that has measurable phosphate and green hair nuisance algae on the rocks, and you DO want to continue using other filters and water changes, then you can just use the recommended cube-feeding sizes of the scrubbers. Use stronger light if possible because the higher phosphate in the water needs brighter light to make the scrubber grow green. And if you double the amount of scrubbing (two scrubbers instead of one), the problems will clear up twice as fast because there will be twice the amount of algae absorbing the nutrients out of the water.
7) If you are adding a scrubber to an existing reef tank that has measurable phosphate and green hair nuisance algae on the rocks as in #6 above, and you DON'T want to continue using other filters and water changes, then double the amount of scrubbing recommended in #6.
8) If you are adding a scrubber to an existing reef tank that has NO measurable phosphate, but has LOTS of green hair nuisance algae on the rocks, then you need the strongest lights possible (for your scrubber size) because the rocks are already full of phosphate, and the algae on the rocks is absorbing this phosphate, meaning you need the strongest scrubbing possible in order to out-compete the algae on the rocks. This is the hardest situation to fix, so you should use as much scrubbing as possible with the strongest light available (for the scrubber size), and use as many other filters and water changes as possible too, until the algae on the rocks turns yellow and lets go. At this point coralline will start to cover the rocks, and you could consider selling some of the extra scrubbers, or removing the other filters or water changes.
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