I'm new around, so please don't pull out the guns!
I thought I will start a clear separate topic only on Theodoxus, so it can be dealed with it separately from other Neritid species.
Allex, Anna and the Deutsche Herrn ( with a long nickname), thank you for your interesting posts. Here is what I can contribute with:
a. Theodoxus fluviatilis can reach very large densities in nature. If you are in the right place you can collect it by pounds. Their shell design is splendid and they go perfectly with our shrimp tanks. Long time ago I've seen huge populations in the Danube Delta. Took some home and they breed nicely in plain tap water (average hardness).
b. They can be collected in land locked freshwater habitats, so no need for any marine stage or such.
c. They were reported also upstream Danube into Hungary, Austria and Germany.
c. For the Swedish members there is a paper on Theodoxus which mentions thousands of individuals per sq meter. I just emailed the author for a copy of the full article. The abstract mentions the name of the lake they studied, so maybe some of the Swedish members live in the vicinity
d. I will do a scientific database search and report back on findings.
e. As Anna implied, Theodoxus can become an obsession!
f. Does anyone keep Theodoxus in US?
kind regards to all good people around
dami
Very nice picture, I am thinking about keeping a snail only planted tank with the many different types of snails out there. I will need to figure out a way to control the population as we know how it can boom with overfeeding 
