I Need Help With My Kind Of New Tank

I have a 65g tank I currently have 1 pleco 2 rainbow shark and had 4 guppies but recently 2 of my guppies died. I check for wounds and missing pieces of fins but I can't seem to find any I tested my water and everyrhing was in normal range except my hardness which I'm not sure how to lower. Also my tank seems to have some sort of white pieces of I don't know what that seem to settle in the gravel substrate and any sort of movement seem to lift it up and make the water cloudy. Also I was adding a bubble stone for my air pump but it makes the water move a lot and I'm not sure if its too big or its normal the guppies seem to get wild whenever I turn the pump on. I would appreciate help as I am new to the big fish tank scene. Thank you in advance

 

Could you add a picture of the white stuff? Other people would definitely be able to give you a better diagnosis with a photo.

 

Could you add a picture of the white stuff? Other people would definitely be able to give you a better diagnosis with a photo.

 

If the tanks new sometimes guppies just die when introduced to a new tank (It’s happened to me)

 

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