I cannot be definitive here, but I do recall a thread a few years ago that dealt with this same issue and the cause was put down to the substrate being Fluval Stratum. I just looked this up on the Fluval website, and I must say they do make some rather worrying claims. And maybe ammonia or nitrite, whichever, is a result.
- Porous structure allows for rapid colonization of beneficial nitrifying bacteria
It may well, but why? The last thing you need in a planted tank is nitrifying bacteria to compete with the plants for the uptake of ammonia/ammonium. This bacteria will obviously appear, but encouraging it--no.
- Encourages strong plant growth – roots easily penetrate and spread throughout substrate to obtain key nutrients
Maybe, but what nutrients specifically? They do not mention any that I can find.
- Helps support neutral to slightly acidic pH – ideal for most plants, tropical fish and shrimp normally kept in planted aquariums
How exactly does it mess with the pH? Not something I would want occurring, for unknown reasons.
- Will not discolor water and helps control organic discoloration when natural driftwood is present
Another unknown that could cause trouble for fish, depending how this achieves it. If it just contains carbon, not a problem in itself but carbon removes dissolved organics that are plant nutrients.