Watts per gallon rule for lighting

Discussion in 'Lighting' started by funkman262, Mar 20, 2010.

  1. funkman262

    funkman262Well Known MemberMember

    I'm still trying to learn about lighting for plants. First off, I have some Amazon Swords, a few Anubias on driftwood, and Micro Sword spread along the tank:

    I'm using the 36" Freshwater 78 Watt T5 fixture from this site:

    I have a 45g tall tank (maybe 35 after sand and decorations) so here are my questions:

    1.) I've heard about the WPG rule to determine if your plants have enough light. What type of lighting does this refer to? I believe if you're getting 2 wpg using incandescent bulbs, that's not as good as 2 wpg using T5bulbs.

    2.) Does it seem like my plants are getting enough light with this fixture or should I get plants that tolerate lower light?

    3.) Based on the bulbs that come with the fixture (one 39W 10,000K and one 39W Freshwater), should I switch them out for bulbs that would give me better results?

    Thanks all ;D
  2. mathas

    mathasWell Known MemberMember

    That depends on who you ask; I've heard some say that T12 fluorescent was the basis for the "rule", and I've heard others say it's T8.

    In general, I find watts per gallon to be about as useful a metric as inches per gallon when talking about stocking... sort of vaguely helpful as a very general guideline, but not terribly useful at all when applied to a lot of specific situations. There are just too many factors involved in the lighting/plant relationship to boil the entire equation down to just two variables and get any type of meaningful result.

    You are correct, though, that T5 bulbs tend to provide more usable light per watt than incandescent.

    If your current plants are not showing any signs of damage, things are probably fine.

    Your bulbs are fine. You might be able to pick a slightly better bulb than what you have now, but the improvement is likely to be so small as to be unnoticeable.
  3. OP

    funkman262Well Known MemberMember

    Thank you mathas :). I've heard in other posts that you are the person to talk to about lighting so I'm grateful that you were able to answer my questions. I'll just keep a close eye on them and monitor the plants' health.
  4. Nutter

    NutterFishlore VIPMember

    Excellent advice from Mathas, I fully agree about the usefullness of the WPG rule. It's a very rough guide at best IMO. I believe the WPG rule is based on the T12's but as Mathas says, others say differently. I think so long as your plants stay healthy & start growing soon then your light is fine. The next month or so should tell you if there are going to be any lighting issues.
  5. OP

    funkman262Well Known MemberMember

  6. _Fried_Bettas_

    _Fried_Bettas_Well Known MemberMember

    There are many variables that make huge differences, to name a couple, the reflector used and distance the light is from the surface of the water. Like Nutter said experimentation will give you the best idea where you are at.