Do you have no choice but to use hard water? If so, look for 'hard water' nutrients. They are formulated with fewer of the minerals, such as bicarbonates, normally found in hard water.
As mentioned in an earlier article, you want to use the best water you can obtain. But sometimes there is no avoiding tap water. If this is the case, do what you can to make the water more suitable for improved hydroponics.
Another alternative is to use quality organic mediums and 100% organic nutrients.
Remember, unlike growing in soil, there is nothing to buffer nutrients and imbalances that may be caused by hard water.
Organic products tend to allow beneficial nutrients to be delivered while holding less desirable nutrients or excess imbalances away from the plants. This will produce bigger yields that might otherwise be diminished by poor quality water.