From the information you've given, this occurred due to lack of nutrients.
Plants need 17 nutrients every day in order to photosynthesize (grow). While presumably they can go a few days without this or that, eventually it will take its toll. Plus the fact that they had nitrogen and carbon (CO2) from the fish throughout plus light, which means they were prevented from photosynthesizing by a lack of other nutrients (iron was mentioned and is important, but so are the other 15 minerals). Yellowing leaves is a sure sign of nutrient deficiency, though it can occur from other things less common.
Flourish twice a week is supplying a good source of most all the necessary nutrients; stopping this is bound to have an effect. Over a period of one year, I experimented three times with reducing my weekly dose of Flourish from twice to once. In every case, within 1-2 weeks, the swords began yellowing. I increased the dose back to twice a week, and within 1-2 weeks the yellowing ceased and new leaves were and remained green. [The "yellow" leaves never recover as Austin or someone correctly said, so remove them.] This I think shows how quickly the increasing or decreasing of nutrients can affect plants.
To other comments in this thread. All plants assimilate nutrients through the roots and some through the leaves (different plants do this more or less). In both cases however, the nutrients come from the water. Solid nutrients in the substrate will have no effect if water doesn't get down there to dissolve them and carry them to the roots. Land plants works the same, so this is no surprise. Good water circulation through the substrate is absolutely essential to provide nutrition to plant roots and to prevent toxic buildup of nitrogen gas. Which is why substrates like sand have to be carefully maintained. It is quite possible to have plant roots suffocate in sand if it compacts.
Where this leads is that liquid fertilizer added to the water will get to the plant roots in the substrate if the substrate has a normal flow of water. Using substrate fertilizer, whether an enriched substrate or root tabs/sticks, works exactly the same but quicker simply because the nutrients are consolidated in that spot where they are most needed--by substrate rooted plants like swords and crypts, that is. Floating plants and plants affixed to rock and wood derive no benefit from substrate nutrients but solely those in the water. I maintained tanks full of swords and crypts for 12 years without any nutrients in the gravel other than what was delivered there via the water circulation (liquid fert), so I know it works. But, having inserted root sticks next to the swords a year or so ago, I did see increased growth in those plants. The point is that nutrients in the water are sufficient, but nutrients added directly to the substrate will result in somewhat faster growth. But the important issue is that the plants are alive and healthy, with slower growth or faster, and for this they need regular and continual nutrient delivery.
Last edited by Byron; 05-29-2010 at 11:45 AM..