I was going to post a caution on the osmocote after Ch4rlie mentioned it, but you obviously remembered my past advice (nice to know someone listens, lol), so I can move on. Will just say the Osmocote issue is because it is a terrestrial plant fertilizer, adding way too much nitrate and phosphate, both of which are not needed in such quantities in an aquarium.
Seachem Flourish Tabs do seem one of the best plant fertilizers. I cannot prove Seachem's claim that these will not dissolve such as to leech nutrients into the water column (comparable to adding liquid fertilizers) but I can say that having a situation in a couple tanks where liquid fertilizers were causing me real issues, the change to just the tabs resolved the algae issues and the plants looked as good if not actually better.
So I have been able to lower the amount of liquid additives (which is always better for the fish, regardless of any other issues). I replace the tabs every two months (Seachem recommend 3-4 months) and now after a year it definitely is working better all round.
The comment from the fish store about fertilizers likely killing the crypt is a bit misleading, as this is certainly true but they do not appear to have explained it very well. Crypts are highly sensitive plants, prone to what is commonly termed crypt melt, where the leaves literally within hours turn into a pile of mush. The roots usually remain alive, and if not disturbed the plant may grow new leaves. Sometimes this takes a few weeks, sometimes months even. The cause can be any change in the plant's environment. Changing the light intensity--and I have seen this from a tube that was left too long before replacement, and adding new tubes; fluctuating or sudden change in GH or pH; change in temperature (several degrees, not what occurs with a normal water change); changing fertilizers; moving the plant or disturbing the roots. A couple species seem stronger to resist this than many others. So the store was on the right track, but didn't cover all bases.
Liquid additives are more of a concern here than rot tabs, in my experience. I use the Seachem Flourish Tabs with two crypts (Cryptocoryne crispatula or balansae), and I saw no change when I started this, and that was after I moved the two plants to a different tank. My other crypts in the same tank, which I think are C. wendtii, do not get root tabs and are doing very well. I do add Flourish Comprehensive and Flourish Trace and Flourish Iron to this tank, but in less than half the suggested doses of each. It is a 90g, so there's lots of space for additives to disperse. This tank is also the one that developed significant brush algae twice just from a second dose weekly of Flourish Comprehensive (without the Trace or Iron at all). The balance can sometimes be delicate.
Byron.
