Jason has covered it, perhaps I may be permitted to expand a little bit.
All plants need a rest period. In temperate regions, they die down to the ground in winter. In tropical regions they slow their growth. I have noted for more than 15 years that Echinodorus species all go through regular periods of growth (new leaves, inflorescences) and periods of little growth (few or no new leaves, and no inflorescences). As best as I can recollect (I don't "take notes"
) this occurs once or sometimes twice during the year. And the extent of the growth phase can vary, from a few leaves to many leaves, one inflorescence to 3 inflorescences, etc.
Second on the light; fluorescent tubes diminish in intensity quite rapidly as they burn. The older T12 gave out very quickly, less than six months (according to Karen Randall who suggested that replacing them after six months was preferable, but staggering them if one had 2 in the fixture would work if they had to go the 12-months distance). The thinner T8 tubes are more efficient, use slightly less energy, and produce somewhat brighter light with less energy; Diana Walstad and some others recommend replacing these every 3 years. But only today I was reading in PFK that all fluorescent tubes should be replaced every six months or at least no less than once a year. I have no scientific data on these opinions, so I mention them for information.
I always replaced my former T12 tubes every year, and that seemed to work. I have let my T8 tubes go longer, they are now in their second year, and some increasing algae makes me think this may be the reason. It is certainly known that when fluorescent tubes weaken to the point of providing insufficient light intensity for plants, that algae will increase. The old balance idea i write about so frequently. Light and nutrients must be balanced for the needs of the plants or they cannot photosynthesize fully. If algae has started to increase, that may indicate time to replace the tube. Personally I would go 12 months minimum with a T8 Life-Glo; my Life-Glo's are now at month 18 but I am wondering...
On the CO2, with the fish you have in a 10g you have no need for CO2. The bacteria in a healthy tanks produces more CO2 than the fish do, and with 7 zebra and a Betta you have a lot of fish in a 10g. [Btw, Zebra danios really should have more space, they are active swimmers, and a 24-inch tank is considered minimum.]