@todvan,
If you can see at least one smooth face on the pink rock Its probably a granite or granite gneiss (The difference is based upon whether the micas seem to be stretched in a preferred direction). If all the rock grains are fairly uniform and angulr nd non-oriented its a granite or a diorite (or one of several other types of ignous rocks (based on the chemistry of the feldspars)
Dont orry, they are quite safe as quarium rocks. The only rocks Id be leery of are those with pyrite crystals (ALL PYRITE HAS EASILY SOLUBILIZED ARSENIC IN IT)or if its heavvily dotted woth iron like magnetite or hematite.
What sort of environment do your fish like? Maybe we can wrk up a recipe.
If you can find rpcks called ZEOLITES, they act like chemical vacuum cleaners and remove cations like copper or arsenic
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