At extreme levels of pH some fertilizers may be less available. 8.4 is pretty high.
Many people in SFBAAPS ( www.SFBAAPS.org )live in your area, and use a blend of RO and tap water to get the KH lower. This allows the pH to drop.
They often keep fish and shrimp that prefer a much softer water, lower pH.
Some plants really thrive in that hard water. They can use carbonates as a source of carbon. I grow the best Hornwort in hard water, and I know Vallisneria is one of the plants that is great for that sort of water.
Most plants are quite adaptable, and do not really care what the pH is. There are a few specialized plants that really need the soft, acidic water. You would not be able to grow those few plants.
There are quite a few fish that would work in hard water tanks, and not eat the plants. So you do not really have to do anything, if you do not want to. Just select fish and plants that are optimum for your water conditions.