I do use Flourish comprehensive, I did get a bottle of Flourish Iron as well. I was thinking that it would be smart to add a little tap water to each water change to boost the GH/KH. I have been hesitant to add more iron because I do not have a way to measure Iron levels yet.
If you use a reliable comprehensive (complete) fertilizer, you should not add additional iron. This is likely to harm the plants and fish. The 17 nutrients plant require need to be proportional to each other. Excesses of some nutrients can cause plants to shut down assimilation of other nutrients. When you are dealing with a fish tank that happens to have plants, the fish must be the priority. This does not mean you cannot add plant additives, but what you add must be minimal.
Going back to the fish you have listed...these are soft water species. With one exception, the knight goby. This species actually does better in slightly alkaline conditions and with sea salt as in a semi-brackish set-up. However, the other soft water species will be harmed by any sea salt, so you have a problem. The goby should not be in with the other fish. There are other very serious issues here too, with the loaches especially. If I get into that this thread will really get involved, so I will leave it and stay with plant nutrition.
You could mix in some source water, to reach say 4 dGH. The soft water fish should have no issues with this. I actually did something similar for a few years, using Seachem's Equilibrium to maintain a GH of 4-5 dGH. After a discussion with a couple of marine biologists, I stopped the Equilbirum, and went to the Flourish Tabs plus Flourish Comprehensive, and the two provided the necessary calcium for my plants without harming the fish.
RO water is fine for the soft water species (not the goby), and plants will be OK as well with a comprehensive fertilizer. Seachem's Flourish Tabs will work for the swords, and their Flourish Comprehensive Supplement for the Planted Aquarium will work for all of the plants. I have basically RO water coming out of my tap, GH and KH are zero with a quite acidic pH, and my plants thrive using the two afore-mentioned products. I've been doing this for over a decade now.
EDIT. Forgot to add that I tried adding iron as well as Comprehensive a few years ago on the advice of probably well-meaning individuals, and within a few weeks my floating plants were melting. The Red Tiger Lotus also began to disintegrate. I stopped the iron, and in a few weeks the plants showed signs of recovery.
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