Blackish substrate around fert tabs! Harmful to fish?

Hello

I am not new to fish, but I have only had plants for about 2 months- so the tank planting is still a work in progress.

I have placed fluorish tabs by seachem in the substrate around swords,crypts, and mondo grass. You could call my substrate coarse sand, or extremely fine gravel. The plants are doing great! BUT ...

The substrate is turning blackish with little tiny particles of black around where I put the tabs. There are 2 tabs that have been in about a month and a half- around those tabs is where I primarily noticed this. I was gently agitating the substrate as I have read to do to check for gas pockets when I noticed this. The substrate is not compacted.

At first I thought I was having some sort of pocket of aneirobic bacteria- (or something like that I read about) until I realized that was around the tablets were placed.
So, I stirred the sand around my newest plants where I put tablets in about a week ago and they are breaking down and creating the same thing! Should I be concerned about the health of my fish
I am primarily concerned about the fish, the plants are just kinda an addition to the tank. So, I don't want to sacrifice my fishes health for the plants.
Sorry for the long post, here are the stats on the tanks in case you're wondering

The tanks in question:
--29 gallon tall w/biowheel 200gph filter, heater, and fluorescent lighting (not really bright, but plants are growing well)
PLANTS: Java ferns, tiger lotus, anacharis, green mondo grass, green and white val, ruffled sword, and amazon sword
FISH: 5 rummy-nose tetra, 6 neon tetra, 5 ottos, 2 panda, 2 peppered, and 2 spotted cory cats
FERTILIZERS USED: Seachem liquid fluorish and fluorish tabs around rooted plants. 20-25% water changes every 6-10 days

--10 gallon w/small biowheel filter, heater, and fairly bright fluorescent lighting
PLANTS: java fern, green mondo grass, some sort of crypt, anacharis, tiny amazon sword, and creeping charlie (I discovered the creeping charlie is a coldwater plant, but so far so good)
FISH: 1 male betta, 4 elegant cories, and 2 ottos (overstocked by some standards, but tank has been up for a while and I haven't had probelms)
FERTILIZERS: Seachem fluorish and fluorish tabs around rooted plants 30% water change every 6-10 days.

Thanks so much, any advice is much appreciated!

Thanks for the reply, John!

It isn't black on the surface, it is just black if you dig down to where the tabs were/are. It kinda smells like sulfur.

But, on the more recent tabs, if I touch the tabs- I can see the blackish stuff flaking off. So, that makes me think it is just from the tabs.

But I still don't know how it might affect my fish.

thanks for the reply. So, you don't think that it could be the anearobic bacteria stuff?

Thank you for the reassurance that hopefully it is safe!