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Discussion Starter · #1 ·

Since many people are saying that Flourish Comprehensive is a better TE than dry fert TE I'd like to know if it's possible to use Flourish with the addition of FESO4 for the Iron (instead of Flourish Iron) ?


Premium Member
6,409 Posts
I second that! freemann loves his iron supplements, and he'd know for sure.

My guess is that it is actually going to work rather nicely. From what I know FeSO4 gives Fe 2+ and SO4 2-, both of which are plant nutrients. Plants take in both Fe 2+ and Fe 3+, but ultimately prefer Fe 2+ over Fe 3+, especially aquatic plants which actually nearly exclusively take in Fe 2+.

I also know that ferrous sulfate (FeSO4) is recommended highly by a number of different sources, and many science labs prefer this type of iron over other sources for plant growth.

However, Fe 2+ is broken down much faster than Fe 3+ in water, especially in the presence of strong light which helps convert it to the more stable form of Fe 3+, so it won't last very long and should be dosed at night when the lights are off to prolong its usable lifespan in the tank.

SO4 2- is also a necessary nutrient, so you would be dosing both at the same time. The only thing to worry about is that you might overdose SO4 and cause a toxicity problem if you dose tons of iron every day.

If I remember right, the last time I saw freemann he was dosing 0.10 ppm per night of FeSO4, though I might be mistaken on this point, he will need to clarify that.

Here is an interesting article that might be good to read: (cached version)

732 Posts
My opinion is that you should avoid FeSO4 like the plague!

Here’s why.

Plants and animals can use both Fe+2 and Fe+3 whatever is available, the problem is that the Fe has to be dissolved in water for it to be useful. That seems like not a big deal but the problem is pH. At the typical pH of your aquarium say 6.0 – 7.5 nowhere near enough Fe is soluble to benefit your plants. This is a problem both for aquatic plants and people who have low blood iron.

The way to correct the problem is to add a chelating agent. Chelating agents are organic compounds that bind up iron and keep it in solution. The classic chelating agent is EDTA. It’s in everything!

The only way you are going to get enough iron into solution to satisfy your plants is to add a chelating agent.

This is a lot of chemistry!

If you check out “Flourish Iron” you will find that they use ferrous gluconate as the source of iron. Here is the part they don’t tell you. USP ferrous gluconate has citric acid added as a chelating agent and gluconic acid (gluconate) itself is a chelating agent. That means they have two chemicals added to the basic ferrous ion to improve its solubility. There is no way you are going to add all those chemical to your FeSO4 and keep anything like balance in your aquarium. It is too much chemistry!

My opinion: if you want to kill everything in your tank doing iron experiments, have fun and play with FeSO4. Otherwise follow a recommended supplier’s plan for dosing and wait until you have your degree in chemistry before you do any experimentation!

The amount of Fe you need is very little and very cheap!
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