Help! What is this spot?

If it's mineral deposits, is it safer for me to use soft water? Does it matter what type of water as long as I dechlorinate it?
 
Depends on the stocking, have you already planned your stocking?
 
What kind of water do you currently have? The mineral deposits are harmless but imo unpleasant to look at. You can clean it off if you like. Make sure the water is dechlorinated or else it will kill off your good bacteria.
 
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