Just looked on their website, and they do make a 15 litre tank. Did the seller say it was a 30 litre? If he did, do you have any comeback for mis-advertising? (Though I don't think that would apply to second hand itmes on bidding sites
)
And I would also make some careful measurements as manufacturers often quote the entire thing as the volume, including the thickness of the glass and the air gap you have to leave on top of the water. It could turn out to be 11 or 12 litres if they do that.
Hmmm, just googled the tank and it comes up on Pets at Home where they do give the dimensions, unlike reef-one. They say it is 32 x 31 x 21 which works out as 14.112 litres - and that will be the total size so the water volume will be 11 to 12 litres. Or even less once you put the media on the bottom.
I'm not sure there are any fish that can be kept in a tank that small, not even a betta
And I'd be concerned about shrimp with the style of filter as baby shrimps will be sucked into the substrate.
However........ Now you have the tank, have good look at it. Would it be possible to change the filter? The manual says it is powered by an air pump. Is it at all possible to replace the whole filter set up with a sponge filter connected to whatever the air pump connects to inside the tank? The manual doesn't show that very clearly. By sponge filter I mean something like
this. This type of filter is highly recommended for shrimps and fry. It would allow you to use a thin layer of sand on the bottom, freeing up some space.
Then get some shrimps.
You would have to cycle a sponge filter of course. But if you could find some way of running it in the main tank for a couple of weeks it would get some bacteria in the sponge, though you'd still need to do a fishless cycle afterwards to build up the bacteria numbers. The good news is that you wouldn't need 3ppm ammonia for just shrimp, 2 ppm or even one would be enough.