There are questions that are routinely asked with respect to screen cleaning. How often should I clean? How much should I clean? When & how should I clean a new screen? How long is the break-in period? So I put this together based on my experience and the interpreted experience of many others. Hopefully this will provide a more general guide to learning the answers to these questions. When is a screen considered "mature" or "cured"? In general, a screen is considered "mature" when algae fills in the holes on the screen. The best way to determine this is to scrape (do not scrub with a brush) the screen fully after 7 days of growth, and then very lightly rinse it. If algae remains in over 50% of the holes (preferably 75%), then I would consider the screen to be mature/cured. In some cases, you can get a lot of thick, green growth in 10 days or less. This doesn't really mean that the screen is mature, but rather that your system conditions are very conducive to Algae Scrubber growth. The following contains mostly information on the break-in period, but a good amount of it also applies to general maintenance: 1) Maintain a moderate flow In general, the recommended minimum flow is around 35 GPH/in (GPH per inch of screen/slot width). Many people report an increase in growth when you increase the flow to a higher level. However, during the break-in period the algae isn't very strongly attached to the screen yet, and too much flow can actually slow or prevent the growth from attaching to the screen. During the break-in period, you aren't going to be really doing a lot of filtering, so the goal not to get a lot of growth, just to get growth started. Therefore, this is the time when you can get away with less than 35 GPH/in of flow. I would not recommend going below 20 GPH/in, and only that low for the maybe the first week or two. After that, increase the flow up to 35 GPH/in over the course of a week or two. After the break-in period, flow can be increased. Theoretically, there is no limit to the amount of flow you can provide, as long as it doesn't cause detachment. As a side note, you might have to increase lighting intensity when you increase flow (this is a relatively new concept) 2) Maintain a moderately intense lighting configuration Just as too much flow can inhibit initial growth, so can too much light. The term "photo-inhibition" has been used in the past to describe this, but technically it is photo-saturation. When there is a lot of light, but not a lot of algae to absorb that light, you can end up with no growth at all. Some refer to this as a "hotspot". This can become evident when LED lighting is in use, but applies to CFLs and T5HO to some extent. It is difficult to quantify how much light is too much, because it depends on proximity, quantity of lamps/LEDs, wattage, etc. The one example of a light source that is almost certain to saturate and cause a hotspot is the use of supplemental Royal Blue LEDs (usually within an array of 660nm Deep Red LEDs). If RBs are run at the same current as the DRs, they will burn a hotspot (which is why I run these at 1/2 current). After the break-in period is complete and growth is consistently strong, lighting can be increased. The upper limit of lighting intensity is still somewhat unclear, and probably depends on a multitude of factors (flow, available nutrients, specifics of tank, etc) 3) Clean slot and top edge weekly. Remove the screen weekly to clean the slot on the pipe and the top edge of the screen (that is inserted into the slot pipe). Make sure there is no algae left on these areas. This ensures that flow continues unobstructed. Ideally, you want to always do this, or at least check the screen often for slot flow obstruction starting with about day 5 (this depends on your individual system, so day 5 is just a number I threw out there) 4) New recommendation: When possible, do not clean the screen in tap water during the break-in period; use saltwater instead. Take a small amount of water out of your tank and place it in a shallow pan, just larger than your scrubber screen. Place the screen in this pan and give it a few gentle swishes. This should loosen any poorly attached algae. For the first few cleanings, this is usually all you need to do. Additional information on this recommendation: Rinsing in tap water kills most baby (microscopic) copepods that have populated the screen to eat algae, which can in turn cause detachment. It's also more convenient than rinsing in saltwater. However, some studies have shown that exposing saltwater algae to freshwater also causes the algal cells to ruture quickly. Also, some users have reported that growth 'recovers' faster after the scren is cleaned in saltwater. Therefore, especially during the break-in period, I recommend only cleaning the screen in saltwater. The exception to this recommendation kicks in when you start to see evidence of detachment caused by copepods. Such detachment is generally indicated by the appearance of circles on the screen where a clump of algae has detached or has been eaten away. This is actually a good sign of a diversity in your system, but it doesn't help your screen. In this case, after you do your saltwater cleaning, quickly rinse the screen in freshwater (tap water, RO, RODI, distilled, etc) and then put the system back into operation. This recommendation applies to all phases of growth in general. Quickly exposing the screen to freshwater every so often is probably not a bad idea, but it should be quick (pods die from osmotic shock very quickly when exposed to freshwater). One way to do this is to shut off flow, pour RODI over the slot pipe, then turn the flow back on. It should be noted that detachment can also occur without copepods. If your screen is not rough enough, or if you have too much flow during the break-in period, this can tear algae off the screen. This usually happens in 'chunks' as well, where you have a spot of excellent algae growth. 5) Let the screen grow! You should not be doing any vigorous scrubbing or scraping of the screen during the start of the break-in period. Your goal is to leave as much algae on the screen in order to encourage additional growth and adhering of the base of algae to the screen. This takes time, and there is really not a lot you can do to speed up this adhesion of algae to the screen, aside from ensuring that the screen is very rough to start with. If you start getting growth that is thick enough to cover up some of the holes, then rub the screen gently with your fingertips - usually, you can just 'swipe' your fingers across the screen and that will loosen a significant amount of algae Typically, you will see growth occur in phases: diatoms (can sometimes look like dinoflagellate), then brown short algae, then green algae. Sometimes you will get bits of red or dark algae growing as well. It is best to just leave whatever growth you get alone. The exception to this recommendation is when you get super-black growth. This doesn't seem to happen very often, but seems to occur in high-nutrient tanks, and may occur in tanks with a certain nitrate-to-phosphate ratio. This type of algae looks like a coating of oil, it is paper-thin, deep black under normal light, and sort of "flakes off" when you clean it. This should not be confused with dark brown hair algae, or long stringy diatom algae that easily rinses away. This type of growth is usually just a temporary phase, but it can completely block all light to the screen, which prevents hair algae from getting the opportunity to attach. This is the only type of growth that should be cleaned off with a toothbrush, and even then, it should be cleaned gently enough so that any other type of growth is not removed. Screens growing the black oil algae should be cleaned as often as necessary to remove the oil algae and allow other more beneficial types of growth to attach. 6) New Recommendation: Clean as needed The rule of thumb for a number of years was to clean your screen every 7 days. However, this is not always the case depending on the type of growth you are getting and your particular setup. Some can go longer, and some grow so thick and so fast that they need to be cleaned at 5 days. Also, many use different harvesting techniques, such as cleaning one side at a time (alternating full-cleanings every other cleaning time), or not cleaning all the algae off of both sides (scrape off strips in a tic-tac-toe pattern on one side, "X" pattern on other, etc). Not everyone gets super-thick growth that would block light to the base of the algae mat (screen), and some use more intense light sources that penetrate deeper, allowing the growth to get thicker before the base of growth starts to die. Flow can also be a factor as well. Therefore, the time frame between cleanings is somewhat subjective, and is going to depend on many particulars of your system. What I feel is a good gauge of how often you should clean is related to how much algae remains in the screen holes after you clean. If you scrape both sides and swish in your pan of saltwater and you end up with less than 50% of your screen holes filled in, you probably waited too long. If it's less than 25%, you definitely waited too long!! Since you can only know this after you actually perform a full 2-sided cleaning, it's going to take some trial and error to determine this. The bottom line is that you will have to learn over time how your system grows algae, and adopt a maintenance program that matches your particular system. I have one screen in particular that I let grow for 16 days or more before doing a 2 sided full cleaning, and there is still over 50% of the holes filled in - but again that was a specific system and I am very familiar with the growth cycle on that system. I would not let the growth go past 21 days without doing at least a partial cleaning (and probably 2 slot & top of screen cleanings) 7) Cleaning a mature screen Once your screen is to the point at which I consider it mature, it will continue to develop growth that is progressively more strongly attached. You will eventually need to scrape the screen with a decent amount of pressure in order to remove growth. This can usually be easily done using a standard plastic pot scraper, which you should be able to find for $2-3 at any kitchen supply store or your big box stores (Wal-mart, Target, Bed Bath & Beyond, etc). You can also use a serrated knife or a hacksaw blade, and some feel that this is better as it continues to keep the screen rough; I don't feel it is necessary to re-rough a screen if it is properly roughed up the first time. I personally only use the plastic scraper, it is much better than a credit card because it is not flexible. You can do partial side cleanings, alternating side cleaning, etc, whatever you feel works best. However, I would recommend a full both-side cleaning occasionally. If this means you clean both sides of one half of the screen, then the other half a week or so later, so be it. But you do want to make sure that the base of the algae mat, at the screen, maintains it's foothold on the screen in order to prevent detachment.