The mortar and pestle could cost six bucks plus postage.
http://www.sciplus.com/singleItem.cfm?terms=8727&cartLogFrom=Search
After weekly cleaning up after a turtle, partial weekly (10-20-25%) water changes where you siphon the tank bottom with the guppies should be a piece of cake.
However, did you "season" the water (by letting changing water sit for a couple of days in a warm spot) you used for the turtle? It would be best if you do that for your guppies.
Maybe do a quick search under "season water".
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