Coral Placement: Light and Flow in a Reef Tank
How to place corals by their light and flow needs: light zones from SPS at the top to softies low down, PAR, flow types, lighting acclimation and spacing to avoid stinging.
Overview
Where a coral is placed in a reef tank determines how much light and water flow it receives, and both must match the coral's natural requirements. Corals from the bright, surf-swept upper reef need strong light and turbulent flow, while corals from deeper or sheltered areas need less. Getting placement right, and acclimating new corals gradually, is one of the main factors in whether they thrive.
Light zones
Corals are generally arranged in vertical light zones. Small-polyp stony (SPS) corals such as Acropora and Montipora come from shallow, brightly lit reef and are placed high, near the light. Many large-polyp stony (LPS) corals prefer medium to lower light and sit in the middle. Soft corals, mushrooms and similar are placed lower or in shade, and non-photosynthetic corals do not need light at all. Light is measured as PAR (photosynthetically active radiation); reef sources cite roughly 450 to 650 µmol for bright SPS placement, with LPS preferring lower levels.
Acclimating to new lighting
A coral moved from a dimmer system to a brighter one can be bleached or burned if exposed to full intensity at once. The standard approach is to start a new coral lower in the tank or under reduced light and move it up, or raise the light, gradually over days to weeks so its zooxanthellae adjust. This is especially important under intense lighting such as metal halide or strong LEDs.
Flow
Water flow brings food and oxygen, removes waste and keeps detritus from settling on coral tissue. Reefkeepers favour turbulent or alternating flow over a single laminar jet, because steady direct current can damage tissue. SPS corals want strong, turbulent flow; LPS corals generally want moderate, gentler flow, since fleshy polyps can be torn by a strong jet; soft corals vary. A common general target is a total turnover of many times the tank volume per hour, distributed so there are no dead spots.
Placement by coral type
- SPS (Acropora, Montipora, Pocillopora): high in the tank, strong light, strong turbulent flow.
- LPS (e.g. brain, torch, bubble, elegance): middle, medium light, moderate to gentle flow.
- Soft corals and mushrooms: lower or shaded areas, lower light, variable flow.
- Non-photosynthetic corals: any light level but high flow and frequent feeding.
Mounting on the rock
Once the right light and flow position is chosen, corals are fixed in place so they do not fall or shift, usually by gluing or puttying the frag plug or base to the rockwork. Secure mounting keeps the coral at its intended light and flow level and stops it from toppling onto neighbours. Final positions are confirmed by observing the coral's response, such as good polyp extension and colour, over the following weeks.