Reef Lighting Guide for Chicago Reef Tanks

Most reef lighting problems we get called about are intensity problems, not color problems. A fixture hung too low, a photoperiod copied from someone else's build thread, an acropora moved from a dim rack into the top third of a bright display: all three show up as tissue loss weeks later, and none can be caught with a test kit. Light is the one major parameter you cannot titrate, so it drifts quietly while the test log still looks clean. Here is the practical half of the subject, from PAR to acclimation.

Reef lighting is measured in PAR, not in watts or brightness

PAR stands for photosynthetically active radiation, the 400 to 700 nanometer band that the symbiotic algae inside a coral can use. The figure quoted in the hobby is PPFD, measured in micromoles per square meter per second, and it describes what arrives at the coral rather than what the fixture pulls from the wall. Watts tell you about your electric bill. Your eyes are a worse gauge still, because a blue-heavy fixture reads dim to human vision while delivering plenty of usable energy below.

Mapping a tank takes about twenty minutes. Divide the footprint into a grid of a dozen points, take a reading at the sand bed, at mid-rock and at the top of the aquascape, leave the fixture at its normal daytime settings, and write the numbers down with the date and the channel percentages. That record makes later changes interpretable instead of guesswork. Diodes dim with age, reflectors dull and lenses film over with salt, so a spot that read 220 two years ago may sit nearer 170 today.

One number still is not the whole picture. Corals respond to the total dose they take in over a day, so a colony at 150 PAR for ten hours receives more usable energy than the same colony at 250 PAR for four. That is why two tanks with matching meter readings grow at different rates, and why stretching a photoperiod by two hours is a real change in dose rather than a cosmetic tweak. Intensity multiplied by hours is the figure that matters.

PAR targets differ by coral group and by where the colony sits

The ranges below are typical hobby targets rather than hard limits, and plenty of healthy tanks run outside them. Treat them as a starting point for placement in a mature system with reasonably clear water, then let the corals refine the plan over the following months. Stability inside a range does more good than chasing the top of it. These are the figures we work to when laying out a new aquascape:

• Mushrooms and most soft corals: roughly 50 to 100 PAR, low on the rock or on the sand.

• Zoanthids and leather corals: roughly 100 to 200 PAR at mid-rock, in moderate flow.

• LPS such as acanthastrea, favia and euphyllia: 100 to 200 PAR, hammers and torches at the lower end.

• Montipora and other encrusting SPS: roughly 200 to 300 PAR on the upper structure.

• Acropora and tridacna clams: 250 to 400 PAR near the top, with stable chemistry to match.

Depth and water clarity move those numbers more than most people expect. Light attenuates as it travels, so a 24-inch-deep display can lose a third of its surface intensity before it reaches the sand. Dissolved organics yellow the water and absorb usable light, which is one of the quieter arguments for running carbon and a properly tuned skimmer. Salt creep and dust on the lenses are worth a monthly wipe on their own.

Corals tell you when the level is wrong, provided you read them early. Too much light shows as pallor at the growth tips, bleaching on the upper surfaces, and polyps that pull in around midday instead of extending. Too little shows as browning, tissue stretching toward the fixture, fading fluorescence and skeletal growth that stalls. Move a struggling colony down or across before treating it for a disease it probably does not have.

Choosing a reef lighting fixture for the tank you actually have

Coverage matters more than brand. Plan on roughly one LED unit per 24 inches of tank length, which means two fixtures over a four-foot display rather than one powerful unit in the center that leaves both ends dim and cooks the middle. Front-to-back spread deserves the same thought, because a puck-style LED throws a narrow cone while a T5 tube spreads light along its whole length. Two modest fixtures beat one large one for filling shadows.

Each technology asks something different of you. LEDs are controllable, ramp smoothly and run for years without a bulb change, at the cost of point-source shadowing under dense rockwork. T5 arrays are diffuse and forgiving, but tubes need replacing on roughly an annual schedule and output is fixed. Hybrids split the difference. Color balance and Kelvin ratings are a separate decision, covered in our guide to choosing a reef light spectrum for corals.

Room conditions belong in the decision too, particularly in Chicago homes that swing between humid summers and dry forced-air winters. A hot fixture over an open-top tank raises evaporation, and in January that means the top-off works harder and salinity creeps if the reservoir empties. Fan noise that seems trivial in a shop is not trivial in a living room. Plan for a dedicated GFCI circuit, drip loops on every cord, and clearance for air over the heat sinks.

Mounting height changes intensity and spread more than the dial does

Most modern LED fixtures are designed to sit between 6 and 12 inches above the water. Raising one widens its cone, evens out the tank and lowers peak intensity; lowering it concentrates output into bright patches directly beneath the optics. Two inches of height is a meaningful change, not a rounding error. Shallow frag systems want more height than their size suggests; deep displays want less.

Hardware determines how easy the rest of your life is. Hanging kits give you height adjustment and clear access for feeding, glass cleaning and coral placement, while rigid arms are tidier but fix you to one geometry. Keep the fixture high enough that salt spray does not coat the lenses weekly, wipe them monthly anyway, and route cables so condensation runs off rather than into a connector.

Change one variable at a time, then measure again. If a tank is running hot on light, raising the fixture two inches is cleaner than dimming to 40 percent, because heavy dimming shifts the balance between channels on some units and changes how the tank reads. After a height change, remap PAR at the same grid points you used before and leave the system alone for a few weeks so the corals can respond.

Photoperiod, ramps and acclimating corals to new reef lighting

A mixed reef does well on roughly 8 to 10 hours at full intensity, with an hour or two of ramping at each end and, if you like the look, a blue-only period at dawn and dusk. Long days feed nuisance algae as readily as they feed coral, so extending the photoperiod is rarely the first fix for slow growth. Consistency is worth more than the exact figure. Run it from a controller and check the schedule resumed after any outage.

New fixtures cause most of the avoidable damage, because corals adapted to one light cannot switch overnight. We work through an install like this:

1. Set the fixture to 25 to 30 percent of your intended final intensity, at the height you plan to keep.

2. Hold that setting a full week and watch polyp extension and color before changing anything.

3. Step intensity up by about 10 percent of the target each week, or every second week on established SPS.

4. Leave corals where they are during the ramp, and reposition anything higher only at full output.

5. Remap PAR at the end and record the final settings, so the next change starts from a known baseline.

Give the process six to eight weeks and watch the corals rather than the calendar. Pale growth tips, midday polyp retraction or tissue receding from the base all mean you are moving faster than the colonies can follow, and the answer is to drop back 10 to 15 percent and hold for two weeks rather than reverse everything at once. If a coral does lose color, our notes on preventing and reversing coral bleaching cover recovery in order.

Key takeaways

• Reef lighting is a dose rather than a setting: intensity multiplied by hours is what corals receive.

• Measure PAR on a grid at sand, mid-rock and top of the aquascape, and log it with the date and settings.

• Match placement to typical ranges: 50 to 100 PAR for softies, 250 to 400 for acropora and clams.

• Cover the footprint with enough fixtures, then use mounting height rather than heavy dimming to fine-tune.

• Ramp a new fixture from about 30 percent to full over six to eight weeks, and move corals upward afterward.

Frequently asked questions

Do I need a PAR meter, or can I trust the manufacturer's chart?

Charts are measured in air or a bare tank, without your rockwork, water clarity or mounting height, so they are a rough guide at best. A meter reading takes twenty minutes and answers the question for your tank. Borrowing or renting one occasionally is enough for most hobbyists.

How long should the lights run each day?

Eight to ten hours at full intensity suits most mixed reefs, plus ramp periods at each end. Shorter days at higher intensity and longer days at lower intensity can deliver a similar dose, but algae generally benefits more from long days than coral does. Pick a schedule you can keep identical.

My corals paled a week after a new fixture went in. What should I do?

Reduce intensity by about 20 percent and hold it there rather than switching the fixture off, since darkness is its own stress. Keep alkalinity and temperature steady, avoid moving colonies, and give the tank two to three weeks. Most corals that pale from a light change regain color if other parameters stay stable.

Work with Marine Concepts Inc

If you are unsure what your fixture is really delivering, we can map the tank with a meter, set a schedule around what you keep and plan the ramp with you, in Chicago or anywhere in the surrounding suburbs. You can browse the lighting, coral and reef supplies we carry and collect them the same day at Marine Concept Supply in Wheeling, IL, where calling ahead confirms what is on the shelf that week.