Rex's Planted Tank Guide

Your Source Of Information For Planted Tank Aquariums, Lighting, CO2 & Fertilizers

Fertilizing the Planted Tank – Nutrients

Nutrients are the foods that your plants need to survive and grow. These include light, carbon (normally in the form of Carbon Dioxide, CO2), nitrogen (normally in the form of nitrates), potassium, phosphorus (normally in the form of phosphates), iron, calcium, magnesium, and many other trace minerals. What this article will cover is everything but the light and the CO2.

There are any number of good commercial aquatic plant fertilizers on the market. Advanced hobbyists tend to use pure chemical compounds to fertilize the tank. To start with you are best off using a commercial product. I recommend the Seachem line, as they are time tested and widely available. For a successful planted tank you need to keep your nutrient levels in balance. This promotes good plant growth and helps to keep algae growth to a minimum. Suggested levels are:

  • Nitrates 10-20ppm
  • Phosphates 1.0-2.0 ppm
  • Iron 0.1-1.0 ppm
  • Potassium 10-20 ppm

If your water is very soft you may also have to provide a source of calcium and magnesium. If you have many tanks and/or a large tank you might want to look for a bulk source of nutrients. I use KNO3 (potassium nitrate), K2SO4 (potassium sulfate), KH2PO4 (mono-potassium phosphate) and Plantex CSM+boron (Plantex is a great trace mineral mix, and we add the boron to it). I also use Flourish Iron and plain old Flourish.

  • KNO3 will dose nitrates and some potassium
  • KH2PO4 will dose phosphates
  • K2SO4 will dose potassium
  • Plantex CSM+B will dose traces and iron

Bulk Nutrients Uncovered

Why would you want to use bulk nutrients. Well here are some numbers.

  • If you are dosing 60gallons of water with Flourish Potassium you would need 100 ml to get to 20 ppm.
  • If you are dosing 60 gallons of water with Flourish Phosphorus you would need to dose 48 ml to get to 1 ppm.
  • If you are dosing 60 gallons of water with Flourish Nitrogen you would need to dose 30 ml to get to 10 ppm.
  • Flourish Potassium is $6 mail order for 500 ml. So if you dose once a week that bottle will last you 5 weeks. Cost would $1.25 a week to dose potassium.
  • Flourish Phosphorus is $7.50 mail order for 500 ml. So if you dose once a week that bottle would last you 10 weeks. Cost would be 75¢ a week to dose Phosphorus.
  • Seachem Nitrogen is $7.50 mail order for 500 ml. So if you dose once a week that bottle will last you 16 weeks. Cost would be 47¢ a week to dose nitrates.
  • Total cost for one dose a week with Seachem $~2.75. Note there is no shipping factored in here and there are some rounding errors.

To dose the same tank with bulk nutrients.

  • Nitrates 2¢ a dose.
  • Phosphates .2¢ a dose. Note that is 5 doses for a penny.
  • Total cost for one dose a week with dry nutrients 6.2¢

Seachem products cost 44X as much per week. Note these prices do not include shipping charges and there are some rounding errors. So that pretty much settles that one.

How Much To Fertilize – Weekly Water Changes

As for how much to fertilize. Well it's not that hard. Say you have a nice tank all set up with around 2-3 watts of light on it. You have the test kits and are keeping the nutrients in the proper ranges. You should be doing weekly water changes both for the health of your fish and your plants. I do 50% water changes on my planted tanks every week

Of course like anyone else life intrudes and some weeks the water changes is missed. But I never go two weeks without a water change. One can use Tom Barr's Estimative Index to help manage the tank. This method relies on water changes to reset the tank on a regular basis. It works quite well. Here is what I do with my 4 wpg 55 gallon tank.

1. Day One

  • 50% water change. Adjust water chemistry. Normally I have to increase my water hardness because my tap water is extremely soft. Dose tank to 5-7.5 ppm of nitrates, dose tank to 10 ppm of potassium, check and dose phosphates as required. Dose tank to 0.2 ppm iron, dose traces.

2. Day three:

  • Test nitrates and phosphates and dose as needed. At this time my tank is chewing through nitrates like there is no tomorrow but phosphates are hardly being used at all. So I'm adding .25-.50 teaspoons of KNO3 every other day to keep the nitrate levels up. Dose 3-4 ppm of potassium, dose 0.1 ppm of iron, dose traces.

3. Day five & Day seven:

  • Same as day three.

Want to know where to get the bulk nutrients? If you live in the U.S.A you can purchase the fertilizers from Rex's Fertilizers or from Planted Tank Fertilizer. If you live in Canada try Hydroponic or you can purchase the fertilizers from my Planted Tank Fertilizer page, you will use a lot more K2SO4 than you will anything else.

For a normal order I would suggest one lb KNO3 (if you need to dose nitrates), 2-3 lbs K2SO4, 4-6 ounces of KH2PO4 (if you need to dose phosphates), and 6-8 ounces of trace mix. Then I suggest you can go to Chuck's Planted Aquarium Site, scroll down to the bottom of the page and download the calculator that will help you figure out how much of these bulk chemicals to dose into your tank.

Note: for those of you who live in the UK I have managed to locate a couple of suppliers of bulk nutrients., Green-Tech and Garden Direct. Those sites should help you get started. If you have any questions about purchasing fertilizers from me, you can contact me directly.