I get a little irritated with Seachem and retailers because they push Flourish so much and a lot of us get sucked into buying it without knowing it is just a micronutrient supplement. When someone sees “comprehensive” on the bottle they are led to believe it has everything needed. Sly marketing but misleading.
Plants need both macro (large) and micro (trace) nutrients for healthy growth. The more you play around with planted tanks you’ll hear things like “NPK,” which is short for nitrogen, phosphorus, and potassium. Carbon is also an important macro but because CO2 injection is a bit more expensive to do a lot of hobbyists don’t supplement that one, or they will dump toxic chemicals like Seachem Excel or Metricide into the tank because - again - clever but misleading marketing.
So you have your big three, NPK, and your micros. The Flourish you are dosing takes care of your micros and some of your potassium (K) requirements but doesn’t provide any meaningful nitrogen or phosphorus which are both extremely important. That’s why a lot of newer hobbyists don’t really see much improvement when they are dosing Flourish and nothing else. If you want strong plants with healthy leaves showing their best colors, you need to first take care of the macros. People with heavily-stocked tanks can get away with little to no fertilizer supplementation because the plants are using the nitrates and phosphates generated by the bio load. When the nutrient demand from the plants is higher than the supply from the bio load, that is when fertilizer supplementation becomes important to keep the plants healthy.
There are generally two ways for properly fertilizing a planted tank. DIY dry ferts (powders that you can order online) and liquid all-in-one fertilizer products. You will run into rabid fans of either method, I don’t really believe one is better than the other. What works better for one person may not work best for another. I personally mix my own dry ferts and have had amazing results but when I tried all-in-one products like NilocG Thrive I ran into issues even though countless others have amazing experiences with the stuff. Best advice I can give is to just pick one and see how it works for you. All-in-ones are simple quick options that take most of the calculations out of the task of fertilizing, but dosing dry ferts allows a person to micromanage how much they want of a specific compound in their tanks. There are several good online calculators like the one at RotalaButterfly that will tell you how much of each of your fertilizers to dose. All you have to do is enter in your tank size and what fertilizer you are using and it will tell you how many grams of that compound to add and how often.
One thing to be mindful of is that nuisance algae thrives off of nutrient imbalance within a tank. If you start dosing ferts and notice an algae problem developing you will need to either decrease the amount you are dosing or increase the other two points of the “triangle,” your lighting and carbon. If you want an easy to maintain low tech tank, stick with dosing just your macros and micros and keep a record of how much and how often you are dosing so you can adjust the dosage depending on when you start to see algae forming. If you want to experience explosive growth that will turn your tank into a jungle within a month or two you can crank the ferts way up but you will need to provide intense light and CO2 injection to allow the plants to use the increased fertilizers.
Start off small and work your way up. You won’t ruin anything. If you overdose your ferts and run into an algae problem just back off the ferts for a bit and do some large water changes to get the levels back down. Please feel free to reach out if you have any questions. I’m no Tom Barr but I have a pretty decent amount of experience with both low and high tech planted tanks and will be happy to offer advice based on my experiences
Happy planting!