How to Determine Watts Per Gallon for Aquarium Lighting

Although fish need a source of light as much as they would in the wild, it's the planted aquarium that needs more care in how much light is provided. With improper lighting--either too much wattage or not enough--plants may not grow as well, or even survive. How many watts you need for your aquarium will depend on two major factors, the type of plants and the tank's volume.

Things You'll Need

Pen
Paper
Calculator

Instructions

- 1
  Identify the species of plants you have in your tank or that you plan to put in your tank and how much light that species needs. Each species of plant will have an optimal lighting requirements, from low light to bright light. To determine the plant species, compare photos on the Aquarium Plants Photos and Profiles page of the Aqua Hobby website, or take samples of the aquatic plants to your local aquarium store for identification.
- 2
  Write down how much lighting each of the plant species in your tank needs. Give each plant a rating of low, medium or bright. The books "The Complete Fishkeeper" and "Freshwater Aquariums for Dummies," as well as your local fish store, are good sources of information for this.
- 3
  Review your list and determine what type of plant dominates your tank. For example, if you have mostly plants that need bright light, then use a bright-light formula. If you have an equal mix of plant lighting needs, then use a medium-light formula.
- 4
  Calculate a range of watts of lighting you need for your tank with one of the following formulas, depending on the amount of light you determined you needed for your plants. For low light, multiply one and two by the number of gallons. For medium light, multiply two and three by the number of gallons. For bright light, calculate three and five by the number of gallons. For example, for a 20-gallon tank with plants that need medium light, you would get a range of 40 to 60 watts (2 x 20 = 40 and 3 x 20 = 60).