It's probably the substrate change, but to quote another person, you're keeping water in this hobby, not animals. Time and time again people will spend thousands of dollars on equipment like fancy tanks, protein skimmers, reactors, lights, powerheads and pumps, etc. only to compromise everything by running tap water.
You can have a RODI up and running for less than $200, cheaper if you're willing to buy a used one and picking up some new cartridges. With the right adapter you can easily hook it up to a faucet and never have to tap into your pipes. Then it's a matter of watching your TDS meters and staying on top of those cartridges every several months.
You might be fine with tap water, but being able to check the TDS meter coming out of the RODI and seeing 0 solves half your problems. Knowing that the source water is not the issue lets you know it has to be something else downstream; excess food, something in your storage container, something dying, etc.
Always a great investment if you intend to stick with the hobby.