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However these are just starting points; a lot of your tank filtering (especially in saltwater) is based on your rocks, so their condition plays a part too in what model(s) scrubbers to get, as well as what type of feeding you are doing, and what other filters you will be using. Here are some specific guidelines:
Since freshwater grows extremely thin, long algae, our scrubber models without strings are recommended. This is because you will probably need to clean the scrubber in your sink with a toothbrush (instead of in-place harvesting while still in your aquarium), and it's easier to brush a flat wall than it is to brush strings. So our flat-wall HOGs such as the smaller HOG1 or HOG1x, or our bigger HOG1.3 or HOG2, work well in freshwater. Overall, if you have the space, the HOG2 is the best freshwater model because it has the largest flat wall space and strongest LEDs but without having strings. For DROP models, the DROP1.2 and DROP1.2x have the most flat wall space without strings. Also, since the thin stringy freshwater algae will flow out of the holes in the scrubber, if you put the scrubber in your display (where the animals are), they will learn to eat out of the scrubber and you will therefore be able to feed less. If you intend to do a large part of your feeding this way, multiple scrubbers will allow the feeding (and filtering) to continue in one when you have cleaned the other.
With saltwater, you can get thick dense growth in the scrubber, which is when strings are an advantage (to hold on to the growth). So any model scrubber is acceptable, and the decision is based on size, and on where you want to put it, and also on how you want to clean it. Saltwater tanks which use live rock (even if the rock is "dead") will need to take into consideration the history of the rock: If it came from a tank with algae problems, each 50 pounds (23 kg) of this rock will add 1 cube a day to your feeding. This is because the rock is really just coral skeletons which absorbed nutrients from the water when the nutrients in the previous (or current) tank were high, and these nutrients will then start coming out and flowing into the water when your scrubber starts working.
After looking at size, the main consideration is where you are going to put it. Since they filter the same wherever they go, it is just a matter of placement. They all produce a little red light which will be visible at night, and some can make air bubbles if placed below the surface, so this will determine if the scrubber will need to attach to your glass (HOG), or float on the top (SURF), or be dropped on the bottom (DROP). Unlike freshwater, the thicker growth in saltwater usually does not flow out of the holes as much, so you can't rely on it for automatic feeding (although you can manually take some growth out, and feed that). And similar to freshwater, multiple units are better than a single unit.
Reefs are the same considerations as saltwater, with the exception that some people like the reef to run as natural as possible, meaning it is filtered by algae alone. With that in mind, here are some more details and options:
1) If you are building a reef tank which is new, where the rocks are coming from the ocean or from a low-nutrient tank with no algae problems, 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 (HOG2 for 2 cubes, etc).
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 (two HOG1 units for 1 cube a day, etc). This will supply the corals and small fish with the most amounts of food particles, and will allow filtering to continue in one scrubber after you have cleaned the other.
3) If you are building a reef tank which is new as in #1 or #2 above, but the rocks are coming 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 tank than your feeding will. So use the 50 pounds of rock = 1 cube of feeding guideline, to add to the recommend scrubbing amount.
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 (SURF4 for 4 cubes, etc).
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, preferrably by having multiple scrubbers which are cleaning alternately. This will keep one scrubber filtering when you have cleaned the other.
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. Choose an Xtra-LED version if possible (HOG1x, SURF2x, HOG3x, DROP1.2x), because the higher phosphate in the water needs brighter LED's to make the scrubber grow green. And if you double the amount of scrubbing (two units 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, especially when you clean one of them.
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 LEDs possible 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 LEDs available, and use as many other filters and water changes as possible too, until the algae on the rocks turns yellow and lets go. The scrubbers for this would be the DROP1.4x, HOG1x, HOG3x, SURF2x, SURF4x and SURF8x.