I don't have frogbit, but I do have fairly dense floating water sprite. I use a submersible pump that I insert by pushing the floating plants out of the way. The pump sits just off the bottom. I pump out half the tank, then refill. No issues.
hmm...a submersible pump...very cool! Perhaps someday!
That density of frogbit will quickly strip water of all nutrients, you will have to constantly lightly fert it.
Why are you bringing frogbit from other tank? Just thin it out to usable/manageable size colony in both tanks and throw excess in trash/compost heap. Reason you see it dying out is 2 fold, colony is crowding itself out and also completely depleting nutrients in tank causing a deficiency which in turn leads to unhealthy growth and premature death of plants. In a healthy managed tank with frogbit kept in check to say covering 1/2-2/3 water surface and proper replenishment of micro/macro nutrients it needs for optimal growth it can clean your tank water so well you almost won’t have to change water or vacuum unless your maintaining a large population of guppy and/or over feeding.
Very good to know! I keep putting it in this tank first, because I have so much from my dirted tank, and second, because this tank is often rather overstocked, so I'm looking to keep the water quality as good as possible. Right now the tank holds just 15ish male guppies, a couple oto's, a couple tetras, a few amano, a kazillion cherry shrimp, a few mystery snails, and a bunch of ramshorns (which were, of course, a mistake). Sometimes it has 10-20 mystery snails and up to almost 30 male guppies (we breed both in order to send them out in science kits for kids to observe). Honestly, I just want the tank happy & healthy -- if that means emptying some of the frogbit, and tossing the stuff from my dirted tank instead of putting it in here, I will happily do so!
As for tank maintenance, I usually remove about 5-6 gallons per week, gravel vacuuming just the open spots in the front, and occasionally at a spot in the center back. I guess I hadn't thought about the plants doing such a good job that I could change the water less! I should test the nitrate & see where it is after a week. Also, I do have some all-in-one fertilizer, but this tank is at work and I have to be careful how much of my own $ I spend on it (boss is not at all interested in spending budget money on extra plants). Generally I add a dose only once or twice a month, instead of weekly.
Thanks for the info on how the plants work -- I had not even thought about the possibility of there not being enough nutrients for that volume of frogbit! I will toss half of it tomorrow and see how it looks next week!