Why light is uneven in a planted tank

The number on a light's box says little about what a plant receives. The same plant can do well in one spot and go thin a hand's width away. Three things set the difference: distance, shade and what's growing above.

Distance from the light

Bar lights send most of their output straight down. One retailer's guide puts the even zone of a typical bar at about 12 inches front to back. In a tank deeper than that, front to back, expect a bright middle strip and dimmer edges.

Height changes the pattern. Raised a few inches, a light spreads wider and softer, and it delivers less at the substrate. Mounted close, it's strong underneath and falls away fast.

Shade from stone and wood

An aquarium light doesn't move like the sun, so a shadow stays where it is. The strip behind a tall rock or a piece of wood can stay dim all day.

Shade from the plants themselves

Plants shade each other. As stems fill in, the carpet below loses light, even though the fixture hasn't changed. It's easy to miss, because a photo from the front looks much the same.

In the one tank we've modeled so far, the model suggests the carpet gets about half the light carpets like, mostly because of shade from the plants above it. That figure is computed. It hasn't been validated against a full set of readings.

What the numbers mean

PAR is the part of light that plants can use, usually given in micromoles per square meter per second. The reading that counts is the one at the substrate, not at the surface.

One published set of bands puts low light at 15 to 30, medium at 30 to 60 and high at 60 and up. Other guides draw those lines elsewhere, so treat them as rough.

How to check your own tank

• Read the plants. Compare the same species in different spots. Stretched, thin growth in one place and compact growth in another tells you more than a spec sheet.
• Rent a meter. Rentals have been posted at about $40 for a week. Dedicated meters cost several hundred dollars. Take readings at the substrate in a grid: left, center and right, at the front, middle and back. Nine readings show the pattern.
• Try a phone app, with care. One owner who compared a phone app with a rented meter judged it good enough, but didn't test it at an angle. Treat it as a guide.

What to change

• Bright middle, dim corners: raise the light a little, add a second bar, or use a wider beam.
• Shadow behind wood or stone: light from two angles. A second bar can fill the shadow the first one leaves.
• Shade from stems: trim them.
• Change one thing at a time. If you change the light and the plants in the same week, you won't know which one did it.

We're building Knowtank to draw this map from a few photos. It isn't out yet.