After water chemistry, water flow is the next most important aspect of a thriving reef. It’s important that every coral, regardless of its ideal flow requirement, has active flow passing through and across its entire surface. The best way to gauge the amount of flow for an individual coral is to look at its growth structure and polyp extension ‘PE’. SPS corals often form networks of tightly knit branches. In order to achieve flow through every part of the coral, they require quite a bit of well distributed, strong current. Dead flow areas can lead to issues like STN (slow tissue necrosis), whereas flow that is too high can damage coral tissue and restrict PE. Often SPS corals will grow into their environment, so once flow for a system is established, it is important to not change anything too significantly. Always observe polyp extension in order to know whether a coral is getting too little or too much flow. LPS corals generally require quite a bit less flow than SPS. It is important to monitor PE, as a LPS polyp will only open part way if it’s receiving too much flow.