Jake’s Planted Aquarium Pages

Watts per gallon ( WPG ) rule:

It is a general rule meant to give a person a good guesstimate as to how much lighting their tank has and consequently what kind of plants they can grow given the correct nutrients and other factors. It is by no means perfect, but it currently the best, most easily attained instrument of measure to give a person a good ballpark estimate as to what plants they might be able to grow. To figure out how many watts per gallon you have, take the total wattage of your lights and divide by tank volume. In a 75 gallon tank with 260W worth of lights over it, it would be 260 divided by 75 equals about 3.5 WPG. That’s all there is to that. Below is a generalized description of WPG = what type of lighting. It’s how I see it, but even so it’s not “set in stone” anywhere, so different people have different opinions as to what is low light, what is medium low, and so forth. Anyway, here is how I see it:

<1 WPG - very low

1 - 1.5 WPG - low to medium low

1.5 - 2.0 WPG - medium

2.0 - 2.5 WPG - medium high ( Co2 injection might be required )

2.5 - 4 WPG - High ( must have co2)

> 4 WPG - Very High (must have co2)

If the tank is very small or very large, the WPG rule means nothing. In small tanks it is because there is a minimum light threshhold. Watts are NOT a measure of how intense your lighting is, but rather how much electricity your bulbs use. Plants desire the intensity of the light and really don’t care about how much electricity your bulb(s) or fixture uses.

Plants need a certain amount of intensity , regardless of the size of the tank. Therefore, in a small tank, if I were using the WPG rule, I could expect to grow low light plants in a 1 gallon tank using a 1 watt bulb. I don’t think I could find my keys with a 1 watt bulb, much less grow some plants. Do you see where I’m getting at? In larger tanks, it’s the same way but opposite , because in many cases you are meeting the WPG rule for a low light tank but with all that lighting you must have to move the WPG , you have the intensity to grow more than just low light plants. To go from 1 wpg to 2 watts per gallon in a 1 gallon tank is 1 watt. To go from 1 wpg to 2 wpg in a 500 gallon tank is 500W. Regardless of tank size, don’t you think the plants would notice 500W worth of lighting before they would 1W ? This is where the WPG rule fails.

In general, the WPG rule as I laid out above is for tanks 15- 90 gallons in size. It still helps to know how many WPG your very small or very large tank is , because seasoned plant hobbyists can still go by it to determine if you need more light or if you have too much.

You can’t just use the WPG rule, because the type of lighting, the reflectors your fixture uses, etc. all ties into how much light is actually getting into the tank. For example, a HO ( high output) T-5 fluorescent bulb going the whole length of your tank is more apt to get light down into your tank than a curly-twisty screw-in mini power compact bulb with the same watts. Why? You get a lot of restrike with the curly bulb, restrike meaing the light coming off of the bulb is getting reflected back to the bulb or hitting another part of the bulb directly instead of down into the tank where it belongs. Likewise, if I had two T5 bulbs in fixtures side by side, and one had a polished aluminum reflector or mylar for a reflector , and the other had white plastic, the one with the better reflector would put more of its light into the tank.

Now, this isn’t something to get obsessed about. I just wanted to point out that not all watts are the same.

To pick out plants that correspond to how much lighting you have or want, go to “picking out the right plants for your tank. ” . The below should give you a better idea of what low light, medium, high light, etc. means to you though.

Low light- Generally speaking, low or medium low light tanks are the easiest tanks to have. The plants which are used for this level of lighting grow slowly and don’t require a large amount of fertilization, and sometimes fish pooh and fish food provides most of what the plants need. Adding co2 might produce some benefits, but it is seldom needed. Pruning plants and other such maintenance can usually be done at your leisure and is usually few and far between. If used, fertilizers are usually limited to a comprehensive fert such as Seachem’s Flourish and maybe some fert tablets for the substrate if you didn’t use a commercial substrate like Eco Complete or Flourite.

Medium light- Medium to medium high are the next step up. At medium lighting, the plants will have an increased need for fertilizer and fish poop and fish food might not get the job done. At around 2 WPG at more, you may start to need to inject co2 , or use a liquid carbon source like Flourish Excel, or you may start to experience algae problems. Also, a lack of fertilization / nutrient imbalance is more likely to cause algae problems at this level. Pruning might be needed every week or two, maybe more for some plants. Fertilization maybe every 2-3 days. You can grow low light plants in this lighting as well, but it’s usually better to mix a few fast growing medium light plants in there as well.

High light - Fertilization a must, typically at least every other or every day when alternating dosing macro fertilizers and micro fertilizers, and algae will result if the fertilization is not sufficient or there is an imbalance / deficiency. Pruning might be twice a week for some plants. Adding co2 is not optional. You can grow medium lighting plants in this as well, but low light plants which grow slowly might attract algae to themselves under intense lighting. Plants such as anubias typically should be shaded if placed in a tank with this much light. Any problems will show up much quicker under this lighting.

Very High light - Good luck with that. Let me know how it turns out.

Tip- The average time people keep their lights on for is 10-12 hours. I keep mine on for 12.