Basics of Flow: One of the most under-rated things to consider when setting up a marine aquarium is flow. Water movement is essential to maintaining a healthy aquarium including: fish only (FO), fish only with live rock (FOWLR) and reef tanks. The biggest purpose of good flow is controlling detritus. With enough flow, detritus is not allowed to settle to rest on the substrate of you aquarium. If it is allowed to settle it will decompose and create nitrates. By keeping all the detritus in your tank suspended in the water column, you leave it no place to go but into the filtration. The filtration cartridges can then be cleaned ridding your system of the nitrate causing detritus. In FOWLR and reef systems having ample flow becomes even more paramount. Due to the live rock being the main filtration in these types of tanks flow to get water to the bacteria is very important. An even higher flow rate than is needed to keep detritus moving is recommended when using live rock as filtration. How much flow? Flow is rated in total tank turnovers per hour. This is the amount of times the total tank volume is run through your pumps/power heads per hour. To calculate turnover rate, use this basic equation: Total flow of power heads/pumps ÷ Total tank volume Example: If a 10 gallon tank had one power head pushing 200gph, then the formula would be: 200 ÷ 10 = 20x turnover rate per hour FO tank flow: FO tanks generally need the lowest flow. From 15x turnover to 20x turnover should be enough flow to keep detritus suspended. FOWLR flow: FOWLR systems will need more flow to get water to the filtering bacteria. 20x turnover is a recommended minimum. Reef flow: Reef tank flow will start out at the same FOWLR guideline of 20x turnover, but depending on coral type, could be all the way up to 100x turnover or more. Coral and Flow: There are many different types of coral which demand different types and different intensities of flow. Many of the beginner corals will do well in the minimum 20x turnover rate and will happily convert to much higher flows to accommodate new additions later. Low flow (20x-30x): Several types of corals do very well in low flow conditions such as: zoanthids, palythoas, some leathers, most large polyp stony (LPS) and many others. Medium flow (30x-40x): Most of the corals from the low flow group will also do well in medium flow. Medium flow also adds a few very fast growers such as xenia and more species of leather coral. High flow (40x-100x+): Some of the low and medium flow corals will do ok if placed out of direct flow up past the 50x turnover mark. Corals from the medium flow group such as: xenia and leathers will still thrive in high flow. Fleshy LPS will have to be placed in a relatively dead spot to survive in a high flow aquarium. 50x+ turnover introduces a whole new group of corals known as small polyp stony (SPS). SPS coral needs very high flow to keep its polyps clear of waste. Type and Direction of Flow: FO and FOWLR tanks will do well in most types of flow as long as there is enough. Reef systems however demand more chaotic flow to keep coral healthy and growing. To create a more chaotic flow for your corals you can position power heads so that flow converges in one spot, set pumps on timers or use a wave maker. Laminar Flow: Laminar flow is unidirectional. You will usually end up with this type of flow when you have only one source of flow. Very few corals such as sea fans thrive in this type of current. You can get away with laminar flow in a FO or FOWLR, but it may lead to detritus settling in dead spots. Alternating Flow: Alternating flow is most comparable to a wave motion. The water pushes forward then back in a rhythmic motion. This is the type of flow you will get from a wave maker or some timers. This is a good flow for all around coral health. You can push a lot of flow in an alternating pattern without injuring LPS and other low flow corals. This flexibility with pushing a lot of flow while it also being healthy for low flow corals makes this the best choice for a mixed reef. Random Chaotic: Random chaotic flow is usually the best choice. It is easily achieved with the use of several power heads pointed to converge in the tank. This type of flow is contently changing and thus does not beat the coral from one side all the time. Because corals are more commonly being hit with full flow from at least one side at a time, the usually restrictions on what types of corals will thrive in higher turnover rates will apply.