I had a successful 21Ltr local marine (mudflat) tank,
the 2 key points I found helped was using live sand, and you need to get to the black stuff underneath, its full of de-nitrifying bacteria.
tip 2 is choose your species for hardiness. triplefins and gobies are awesome, most of your common shrimp species are good too
I ended up shutting it down as this summer (new house) I wasn't able to keep it as cold as before and was starting to lose the cockles in it.
I Cycled for a month with just the live sand, then added some sand gobies, then a couple of weeks later some rocks (with associated livestock)
the shrimp were a whim about a month later and did well.