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Need your opinion on an MH light.

Discussion in 'General Plant Topics' started by ntino, Oct 16, 2008.

  1. ntino

    ntino Guru Class Expert

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    The tank is 24x24x24, lighted with a 250HQI light hanging from the cieling.
    When getting the light, I knew it was alot of light for the tank, even though it will be a high flow high co2 tank. However, I wanted the option of playing around with how high I put the light up.
    After setting up the tank and the stand, I realised that the highest I can have the light up from the surface of the water is around 18".
    Do you think this will be a problem?
    There wont be any tall plants, and I dont think it will have any stem plants at all(HC is stem I guess, but that hardly counts).
    Those of you with a PAR meter, how tall do you think my light needs to be? I would rather not downgrade to 150w.
    Another thing I havent planned for is that the 250w hqi runs pretty damn hot.
    I started DSM, and it almost fried all the HC.
     
  2. VaughnH

    VaughnH Lifetime Charter Member
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    HQI bulbs are close enough to being point sources of light that the light intensity from them drops off with the square of the distance from the bulb. So, the drop in intensity from when the bulb is 2 inches above the tank to when it is 18 inches above the tank, assuming 2 inches of substrate, is about (2+24-2)/(18+24-2) squared, which is .36 (36%). That means the intensity with the fixture raised 18 inches, at the substrate in the tank, is about 36% of what it would be if the fixture were only 2 inches above the tank. 250 watts over 60 gallons is about 4 watts per gallon, but raised 18 inches it is equivalent to about 1.6 watts per gallon located 2 inches above the tank, where a standard PC light would be located. I think the 250 watt bulb will work fine. The only disadvantage I see is the light spill over from raising it that high.
     
  3. Tom Barr

    Tom Barr Founder
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    Yes, that's a lot.
    You can swap the ballast/lamp holder and go to 150w etc. Or run the MH for only 8 hours at most.

    Consider only 4-6 hours for the DSM/or use a 24" FL bulb for now.

    Regards,
    Tom Barr
     
  4. ntino

    ntino Guru Class Expert

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    I raised the light up as much as I could, and it seems to be ok for now, problem is that I started off with not such a healthy batch of HC, hopefully they will recover, but I had to store them immersed for a little while I was away for a few days, so some started melting off.
    Other than the humidity factor, can there be too much light for DSM?(I would imagine not, but not sure).
    Assuming high CO2 and good dosing of ferts, I could go 10" from water and around 9 hours aday?

    Reason why I wanted to keep the 250W is that I really wanted to see how fast certain plants and moss can grow under optimal conditions and really high light, and how much I can push it without algae.
     
  5. ntino

    ntino Guru Class Expert

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    On the same subject, what do you think of planting a partly melted HC when doing DSM(due to long shipping and the plants spending several days in another tank water before being planted)? I have experience in growing submerced HC, but I have always started with healthy HC to begin with, so how fragile is HC?
    the plants look like they started melting, but some of the leaves look decent, while most of the little bunches of stems I planted are partly yellow/brown? Will it recover or should I buy another batch?
     
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