Regular flourish? It will either just say flourish or comprehensive.
That shouldn’t cause issues, as it’s just micronutrients (careful not to overdose as there are things in there the shrimp won’t like at higher concentrations.)
What can cause Val melting is labeled excel or co2
booster. If you dosed either of those it could definitely cause the die off. I’ve used excel with vals but they need a little babying to work up to the dosage; hitting it with it all at once or overdosing can make them grumpy.
Otherwise, did you do any kind of dip or rinse etc that might have upset the vals before planting. I did a hydrogen peroxide dip on mine once and they seemed fine at first the. a few days later melted down to roots! Whoops!