Lighting in 75 Gallon Tank

[jptjpt]

Are your other plants crowding out the L arcuata? I'm surprised that it would not grow rapidly in your setup. I have a 55G (same height as 75G) w/two PC 55W bulb (retrofitted AH supply kit into an old 48" fixture) sitting right on the tank rim. Tank has mineralized soil substrate and pressurized CO2. The L. arcuata used to grow like weeds that I ended up pulling it all out. I did notice the leaves at the bottom dropping off when it got overcrowded.

I also have a 75G with the Catalina fixture (4 bulbs instead of 3). The tank has mineralized substrate but no CO2. I only keep 2 of the bulbs lighted for 6hrs, 1 hr off, then 2hrs on. I had moderate success growing L. arcuata in this tank. It grew well, but a little slower than the 55G (probably b/c it's missing pressurized CO2). Even with 2 bulbs (6700k + 10000K) I'm getting BBA off and on. You should be able to keep that 3rd bulb on with CO2. Just keep a closer eye on the tank as it's harder to manage with the higher light output.

BTW, both tanks get DIY root tabs and Fluorish K periodically.

 

[rockhoe14er]

2.16 watts per gallon of T5HO is a ton of light. It will be hard to put enough co2 in your tank to support that much light. I have two 24 watt T5HO lights 6 inches above my 29 gallon tank and they are close to 24 inches above my substrate and measuring with a par meter i'm close to 100 par at the substrate and 200 at the surface. Since this is pretty high light i have to crank my co2 up to around 40-60 ppm to provide my plants with enough co2.

just checking out this thread it has great information on lighting
http://www.plantedtank.net/forums/lighting/105774-par-vs-distance-t5-t12-pc.html

Also according to this if you have 3 T5ho lights 17 inches above your substrate that puts you at 225 par at the substrate and probably 450 at the surface. That is just a ton of light. I would raise that fixture with all three lights on about 12 inches above your tank to put you in the 29 inch range and you should be good to go and have close to 90 par at your substrate.

 

[cbster]

just checking out this thread it has great information on lighting
http://www.plantedtank.net/forums/lighting/105774-par-vs-distance-t5-t12-pc.html

Looking at that i have way to much light with 2 bulbs going so why would there not be leaves at the bottom of the plants?

 

[Yoemen]

Not sure if it will help but:

I went from having 4 shoplight bulbs on my 75 and growing DHG great, then switched to a 4x55 watt from AHsupply and was all of a sudden couldn't grow DHG for anything. Finally figured out that I had the fixtures too spread out and that too much light wasn't reflecting into the water properly. Problem solved.

just an idea, took me 12 months to finally figure out the issue.

 

[rockhoe14er]

The other thing i have recently realized is that reflectors play a huge part in these lights. If you have a bad reflector then you could be getting really low light or if you have a really good one then you might be getting too high of light. A par meter is really the only way to tell.