Do I Need To Have Surface Agitation For Oxygenation.

Hi Fiona.
I took a bit of each out of the tanks. The "frogbit" is on the left and the "water lettuce" is on the right.
Apart from size, leaf thickness etc., you could also have a look at the roots. My frogbit seems to have long, stringy roots, and the water lettuce has branched, feathery roots.
Not sure how much this helps.
 

None of these look like the true water lettuce which is at least as big as the frogbit, if not quite a bit bigger. I had lots of it in my young fishkeeping days when it was the only floating plant generally available. The small one might possibly be Salvinia natans.

Again, the perils of using English names ................

The plants were considerably bigger when I first had them Richard, the outer leaves rotted off and the smaller plants were connected to the larger by a threadlike stem but they all broke off too.

Well I've found some true amazonian frogbit on amazon ironically, so that'll arrive next week. Shame I can't use our native frogbit, I have tons of that starting to grow again in the pond.

Thanks for all the help folks

I've seen that for sale and it looks nothing like mine. There are water lettuce described as a dwarf variety. However I've just found this on a website: "Growth depends upon conditions. The so-called ‘Dwarf’ Pistia stratiotes is not a distinct species but simply due to less-than-ideal conditions of light and nutrients. Dry, curling ends of leaves are typically due to low water vapor. Also it is badly affected by drops of water formed by condensation on the lid of the tank."

So I'm guessing mine is stunted from less than ideal conditions

Fiona, I'm sure yours isn't water lettuce. The true water lettuce does suffer from dried out leaves, because they stick up above water level. I think it used to do better when we had different lights which were further from the top of the water. With most tanks these days there is very little gap between the tubes and the water surface which means that the leaves are very close to the hot tubes. Maybe it's a plant that would do better with LEDs which give off very little heat.

Looking at the pictures on the Tropica site you can see how it gets its name, the sticking up leaves do look a little like a lettuce, whereas those plants with leaves flat on the surface look nothing like any lettuce I've ever seen!

I don't think the LFS shops are any better than EBay etc for getting the names correct.