I've been looking at a tiny pond made from a sawed off chemical tank, in a garden and noticed that there were strange protrusions from the walls. The walls were covered in aglae but there were snail trails in the algae.

Now I remember that last winter I threw in some pond snails I collected from Chawick Lakes, but what I saw was definately not pond snails. A quick search seems to indicate these are 

Ancylus fluviatilis.

Has anyone seen this locally before? I wonder how it got in this pond.

has anyone grown them in aquarium?

Well, as your link says, these are not limpets but snails. They are ramshorns that developed their shell that is similar to that of limpets. 

This link provides better photos, and you can notice that it is actually a snail.  Most probably you got them the same way we all get pond snails in our ponds.

http://www.molluscs.at/gastropoda/freshwater/freshwater_limpets.html

u le x ramshorn snails loooooooool. lewwel net ramshorn memmx chadwik lakes u li tefa fil pond tighu huma sempliciment tadpole snails li issibom kullumkin li alija jin dawk pests. jista jaghti il kas illi il freshwater limpets kin em xi ftit bajd imwahhal mas snails u dawn mumix komuni hafna gewwa malta. pero naccerta illi huma 100% Ancylus fluviatilis. thanks

Ma naħsibx illi fhimt il-punt tiegħi u lanqas naħseb li qrajt l-artiklu tal-link li tfajt.

Fresh water limpets are common in many streams of Malta. Completely harmless. But not really interesting aquarium subjects.

---The river limpet (Ancylus fluviatilis) and the chamber limpet (Ferissia clessiniana) have evolved from the ram's horn snails (Planorbidae)---

Did not know that. Very interesting. If these 'limpets' are related to the ram's horn snail they could make part of the clean up crew in an aquarium. Couldn't they?

Since i saw them I have set up a small cold water aquarium, it is still empty except for snails. I will try to move some limpets there and see what happens, I huess in summer they will die from the heat and I hope they don't turn on the plants.