Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
In general plants that float need some place to stop moving. When the water pushes them into corners or starts jamming them together this gives them the stability they need. One way to get them started is to make a ring of air tubing and connect it to the side of the tank with a suction cup. A ring as small as 3-4" is a good start for the smaller plants.
Some plants are better at holding a position when they are anchored in simple ways, and can therefore stand up to more water movement.
Water Lettuce, grown as the upward growing habit that is common in higher light open top tanks and ponds will handle quite a bit of water movement once it has found a stable spot. It will send out short runners and grow new plants around the original one, and the secure original plant will provide stability for the new plants. Example: 5' x 16" tank with 2 Fluval 404 canisters. Water Lettuce got pushed under the rim (acrylic tank) until it sent out runners, and the baby plants on the runners got so strong they grew right up to the outlets of the filters.
Water Hyacinth will grow in a similar manner to Water Lettuce. Let even one plant get a good hold and the runners will spread even into some pretty strong water movement. My neighbor has some in a stock watering trough that keep taking over. He has it set up with a pretty strong pump at one end, and the WH fill the trough very quickly from a secure place at the quieter end.
Azolla meshes together, and looks sort of like overlapping scales, or roofing. But it does not like water movement. It grows in the wild among cat tails and similar growth that greatly reduces the water movement. Once it has grown in good and dense the water can start moving faster, and the dense growth habit will hold it together and it does fine. In general, I would not call it great for a fast moving surface.
Duckweed does not seem to form any sort of secure attachment and in a tank with too much surface movement it may hover in the calmest corners, but does not thrive. A ring of air tubing can help, even if this ring drifts around the tank. This ring could be anchored with a suction cup in a manner similar to described above.
Frogbit can handle quite a bit of water movement, for example Aquaclear 110 filter on a 4' x 18" tank with a Koralia #4 under the water. The frogbit started doing pretty well where it grew just tall enough (yes, above the water surface) to stick up at the back of the tank. The frogbit grew between the glass lid and the tank. Then it sent out runners from that secure position, and it sent roots down to the substrate. The roots are not doing anything to hold it in place. I separated it to share with some friends, and only the runners overlapping and meshing together provide the stabilizing base from which they grow.
Red Rooted Floater: I just got more of this today, for a higher light situation. In the past RRF did not really do well in my lower light tanks, behaved about like duckweed, with no connections, just drifting around... and around...
Riccia does not seem to do well floating with water movement. It grew really well on the water fall part of the HOB filter. When I took that tank down to move it I placed the Riccia in the water, and kept the duckweed in the tank, too. I think the Riccia needs time to find each other and to anchor together. It will grow into a raft, but it will do this a lot faster inside a ring as described for duckweed.