Provided you do not overdose, Flourish Comprehensive will not harm invertebrates (shrimp, snails) or fish. The level of copper (which is usually the mineral most are concerned with) is not anywhere near high enough to harm them, again provided you do not overdose.
You might have more copper in your municipal water than in Flourish Comp. Not suggesting you do, but we often forget that municipal water may contain heavy metals like copper, and the level of copper allowed by law in water is safe for humans but not for fish.
Generally with this product, use less and increase only if the plants show signs of needing this. The balance between light intensity and nutrients is sometimes finicky and may take a few weeks to sort out. But the advice of people like Tom Barr that one should always start minimally with fertilizers and increase only to the level needed is sound. I have had brush algae suddenly appear solely from using a double dose of Flourish Comprehensive; when the dose was halved, the algae disappeared. This occurred twice, and the second time I controlled the experiment, so I know it was accurate. I have found a partial dose of Flourish Trace actually benefits the plants more that using a full dose of Comp. I use both, but at less than recommended doses.
The substrate tabs are ideal. To lessen the liquid additives, I am now replacing the tabs every two months (they recommend 3-4 months) and the plants are thriving. And I have very soft water, so these alone are supplying sufficient hard minerals which had been lacking previously. I use the tabs next to the larger plants, like swords, lotus, aponegeton. I do not use them dispersed, just one next to a plant.
The other thing to keep in mind is that the liquid plant additives are in the water and thus inside all the fish. Less is always better for the fish.
Byron.
