I have a 55G tank with two 54W t5 lights. The tank is 24" deep. My question is how far down can the lights reach to still give the 2 watts per gallon rule? Is the bottom of the tank getting the entire wattage?

Thanks

No, the bottom of the tank is probably not getting 100% of the intensity the bulbs are putting out.

Are they T5HO's? If so, the Watts-per-gallon rule doesn't really apply, ESPECIALLY if the two bulbs have independent reflectors, because the lights are so efficient. The WPG rule was originally intended for a T8 fixture.

The true measure of whether your plants are getting enough light is a PAR light meter, but they are pretty pricey. I've got a 48 Watt T5HO fixture with independent reflectors over a 29 tall with a 24" depth, and I'm getting low light at the bottom of the tank. The factors affecting this are #1 plants growing between the light and bottom that shade the bottom, #2 the interference of things like a dirty splash screen, or my incorporation of a full glass Versa-Top, and #3 the presence of scum on the surface of the water.

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Yes, my lights are two T5HO 6700k. I have a DIY reflector/housing made up of a piece of round air duct cut in two with a piece of plexiglass. Not sure if that is reflecting enough light into the tank though because my plants are not growing much. I dose with fertilizer every so often and have a DIY CO2 system.

Your light sounds like it's just fine - 100 Watts of T5HO over a 55 gallon is medium-high light. I've got 48 Watts over a 29 and my plants grow like weeds.

What are you using for substrate?

DIY CO2 isn't very effective once you get into the bigger tanks like 55 gallons. How many bubbles per second are you pumping into the tank, and what are you using to diffuse them into the water?

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My substrate is medium sized gravel, but some of my plants I have put into pots with FluVal plant substrate. Bubbles per second is around one to two and i diffuse by pulling the bubbles into the filter intake. I am not big on the CO2 thing but figured I would give it a try to see if I noticed a difference in plant growth.

Hello Vik...

You have essentially 2 watts of light per gallon in your 55 G. If you're not planning on keeping lighting demanding plants, then your T5s will allow you to grow most aquatic plants.

I really don't think you need to be concerned with the amount of light getting to the bottom. The T5s will generate a lot of light over a small area and you have two, so the intensity would be enough for most plants, even those that remain fairly short.

I would take this a step further and dose some type of fertilizer and you'll have a well planted tank in no time.

B

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Low Tech Water Building Keeper

Agreed with BBrad - you have medium light levels, and high light levels near the water's surface. I would look into some fertz (or just better substrate besides potted Fluval substrate, maybe look into Eco Complete), and I would give your CO2 a once-over.

The CO2 system you have needs a good diffuser - the filter will gas out as much if not more than you put in because while it does mix the CO2 into the water, it also agitates the water so more gas exchange takes place. Consider a ceramic plate diffuser put immediately under the filter's outlet. Also, 1-2 bps is a low rate - I put 2-4 bps into my 29 tall to get my drop checker in the green range. Can't imagine 1-2 bps is doing much good in a tank nearly 2x the size of mine.

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Officially fishless until after the move....tune in later for details!