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Living near the Yorkshire Dales I often come back from a walk with odd bits of Limestone, that have caught my eye, in my pocket and these usually end up in the garden. Decided to use one in my tank as a PH Buffer it was then I noticed they're all green on top don't know the type of Algae or if it will grow in Brackish water and if it does will the Shrimp eat it So after a wash in RO water put it in 1ltr with 3gm Marine salt added and under a 6500K light to see how it gets on. Here's a piece that didn't go in the garden, so it's not turned green Crinoidal Limestone

A sedimentary rock with coarse grains which contains the fossilized remains of sea creatures (crinoids). It may be distinguished from other limestones due to the presence of large quantities of fossilized creatures. It is white or grey and contains calcium carbonate which is derived from remains or from sea water. Some calcium carbonate is broken down to form a cement which binds the larger crystals and broken bits of crinoids together. Although it is mainly made up of crinoidea it may also contain some brachiopods, corals or molluscs along with some chert, mud and silt. Crinoids are related to starfish and sea urchins. Today there are over 700 species but those found in calcium carbonate lived 230-600 million years ago.

Great post @QDanT and those rocks looks awesome! There are lot of rocks like that where i live too. Keep an eye on the rock and algae and test for ammonia after a while to see if you are getting a lot of die off.

Did the algae grow in the 1 litre of brackish water and light? Them tiny fossilised creatures are awesome. We only get chalk in Kent.