Do not use iron alongside
Flourish Comprehensive Supplement. Iron is a micro-nutrient, and Flourish Comprehensive contains all that is needed in balance with the other nutrients.
Aquatic plant nutrients must be in a relative proportion to each other. There have been scientifically-controlled tests to determine this.
Flourish Comprehensive has the ingredient nutrients in this proportion. Brightwell Aquatics'
FlorinMulti is the same. Adding any of the nutrients beyond this proportion can cause issues for plants, let alone fish and bacteria. Remember that iron is one of the heavy metals, and these are toxic to life in excess which is why most water conditioners detoxify them--and why liquid plant fertilizers may be best added the day following a water change.
Studies have also proven that an excess of iron causes plants to shut down assimilation of some other nutrients. External signs of a deficiency can be identical to signs of an excess, making it difficult to determine which it actually is. Excessive iron has been shown to reduce tissue manganese levels in some plants [Walstad,
Ecology of the Planted Aquarium, p. 104]. Several years ago, acting on advice that my red-leaf plants would benefit from iron beyond what is in
Flourish Comprehensive, I added
Flourish Iron to the tank, at the dose indicated on the label. Within just a few weeks, my floating Ceratopteris began melting. Since additional iron was the only change I had made, I discontinued the iron. Within another few weeks, the plants that had survived began to show new growth. The red leaf plants (Red Tiger Lotus) also melted but re-grew. There is really no benefit in overdosing individual nutrients.
Iron in excess (= beyond what the plants actually require or assimilate) can encourage algae issues.
Testing the water for iron is not conclusive, and likely leads to an inaccurate assumption. Diana Walstad writes,
The fact that iron is being rapidly removed from the water in your tank does not mean that you need to add iron. Plants quickly take up iron from the water, even though the substrate can provide the iron they need. Therefore, I would not feed your plants iron based on what you measure in the water. [Ecology of the Planted Aquarium, p. 169]