# [APD] Re: how much light?

```This has been a very interesting thread to read, as I have been playing with
light levels in my fairly new 240 gallon tank, which up until a week ago had
four 175 watt metal halide lights about 12 inches above the water surface. My
tank is 24 inches tall. I was having a very hard time with a couple different
types of algae. I played with water flow and altered the amounts of
supplements. I use CO2 injection with controller, and also dose nitrogen, potassium, and
iron, in addition to maintaining GH with Equilibrium and KH with Alkaline
Buffer. A lot of variables to try to see if any of these were helping or hurting
the algae issue.

Anyways, the tank is reef ready and has a sump of about 52 gallons capacity,
which is usually halfway filled. So I'm thinking I have about 270 gallons in
the water column. How wrong I was. I used this volume to get a rough idea of my
light level, which came to 2.6 watts/gallon (700/270). Not bad, I thought.

Here are my errors (IMO):

1. Just because a manufacturer says the tank is 240 gallons, it is not
necessarily right on the money. The best way to get the actual gallons in your tank
is to measure and multiply the tank's inside length x width x height in inches
and divide by 231 cubic inches in a gallon of water. That will give you the

Some tanks are manygallons less than the label may say.

2. I stopped using the gallons in my sump to determine my light level. That
amount of water never sees light anyways.

3. The amount of water in the substrate is minimal and shouldn't be
considered in calculating watts per gallon, as this significantly inflated the volume I
was thinking was in my tank.

So I redid my volume calculation measuring from the top of the substrate to
the top of the actual water level in the tank, and multiplied this by the
inside length and width, and got an actual net volume of 180 gallons, the actual
amount of water moving around the tank at any one time.

This new figure 700 watts/180 gallons gave me 3.9 watts per gallon in my
tank, big difference over the erroneous 2.6 I thought I had.

I removed one of the Halides and now I have 525 watts/180 gallons or a real
light level of 2.9 watts/gallon. The algae has slowed significantly and there
is only a bit left now. I don't think this was the _only_ factor contributing
to my algae problem, but when you are artificially accelerating the metabolism
of your plants, I can only imagine that a very high light level can only make
it harder to manage the other inputs (food-fish poop, CO2, ferts, N, K, Fe).
Plant growth seems to have slowed down a bit, but the glosso is still hugging
the bottom of the tank. I thought it might get taller. I set up the four
halides following the rule of one 175 watter for every two feet of tank length. One
175 watter for every 32 inches seems to be working well.

My actual water column in about 206 gallons, as opposed to the 270 I thought
I had. I'll use this figure if I need to dose meds or liquid fertilizer. I
don't know how much actual water is in the substrate, but not much. I'd think
it's role for dosing or meds as a part of total volume would be rather
insignificant given the slow rate of turnover it is likely to have (just a guess).

Hope this helps.

Dave

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