I think if you were JUST growing plants, and using RO water, you might be concerned about which to use. Most average tap water and fish food has a pretty good assortment of micros and some macros. (Not enough for high tech tanks, but these sources do contribute something)
Think about all the sources of traces and which source contributes what.
Tap water: varies a LOT, and can have almost nothing (very soft water, or RO) to toxic levels of minerals found in some well water.
Fish food (digested by fish or bacteria, it ends up in the tank, available to the plants)
Other: deliberately adding other fertilizer with micros such as tablets in the substrate, or the fertilizer that is already in the substrate (some are quite rich in fertilizers when they are new)
If you know any of your sources are already rich or deficient in something, then get the product that levels out the supply. (Remember that plants use different quantities of each mineral, so 'level' does not mean they are all equal. I mean to make the overall supply of all the micros in a reasonably close balance to what the plants use)
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