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I have used these vitamin & mineral blocks a few times. I have very soft water and have added crushed coral to my tanks via small bags in my HOB filter to bring the GH and KH up a bit to stabilize the pH. I have been testing my water for GH and KH every so often. I did a water change and added one block each to 3 different tanks, but not my 5 gallon tank with only one wet pet. When I tested the tanks after the WC and addition of the block, my water hardness (GH) had skyrocked upwards and knowing the GH as it comes out of my tap this result was impossible. There were no other changes other than adding these blocks. I tested the 5 gallon which didn't have a block in it and that GH remained the same even after a water change. The water in the tanks with the blocks looked yellow. I have since removed the blocks, changed water in two of the tanks and will retest for GH. I called MARS, the Co. that sells the product, explained what happened and the nice gentleman told me that a raise in GH caused by the Vitamin block was impossible. I will continue to test for hardness over the next few days, but I doubt very much that I will be using this product again. Could be a combination of soft water, crushed coral added to my filter and the blocks--however, doubling the GH from 121 to 259 is not something I want to do--it does affect the pH which is why I am using a small amount of crushed coral in order to stabilize the pH. I do not want my pH raising or lowering enough to cause my fish stress. I can't believe that this is only happening in my tanks and if you choose to use the product, I would suggest a check on GH before and after. I am giving this product one star because I am not sure that it did anything for the fishes.