I made a mistake with the identification of my fish. They are actually Three-spined Sticklebacks and not Mackerel fry. A member of the irishfishkeepers.com forum pointed my in the right direction. Thanks Bullfrog.
They only grow to 4-6cm so I might be able to hold on to them which is the good part.
They seem very happy now and not so shy anymore as they are also coming to the front glass even when I am at the Aquarium. Great to watch how they school. The inevitable happened and one ended up as a snack. Actually two Snakelocks shared it 😉 although I think the bigger one won in the end.