Oh yes, our goals are diferent, but I've planted more for performance in the past.
Compared to most ponds, I think your 3-4 gal. Rubbermaid containers would be considered on the large side. I think more people plant in the 1 gal. squat pot more than anything else.
When I started a long while back, I wanted BIG so I planted a number of waterlilies in 10 gal. Lerio pots and went thru large boxes of fertilizer tabs. But I got big plants and many,many blooms. That's when I found out my mentor was using 55 gal. plastic drums cut in half to make two planting pots. Of course, he had a digging attachment on his tractor to lift them in and out of the pond. (A little too hard core for me!)
I've had pretty good luck with the Nutricote product sold as Red Dynomite. I believe it is the 270 day. The label "dose" of 3/4 cap full per gallon works pretty good for my hardies, but the tropicals act pretty much as you describe. Those of mine did slow down and I went to feeding them tabs again. I think increasing the intial dose to a full cap or more would probably work out better.
Now, I'm trying the Pond Pearls. Way too soon to judge this stuff. But over the next few months I'll have a better idea on how well it works. I am liking the idea of once a year fertilizing. I've seen tabs that are supposed to work for that, but I'd rather run one "experiment" at a time. I confuse easily.
Mike S.
Spring Hill, FL