The short answer
No — hard water isn’t bad for fish in general. Plenty of popular species actively thrive in it, and hard water even comes with a bonus: it usually carries plenty of KH, so your pH stays stable and swings are rare. The real question isn’t “hard or soft” but “does my water match my fish?” Some fish love hard water, some prefer soft, and the easiest path is to choose fish that suit your tap water rather than fight your water’s chemistry.
Fish that love hard water
Hard, alkaline water is ideal for livebearers — guppies, mollies, platies, swordtails — plus African rift-lake cichlids, most shrimp and snails, and many rainbowfish. They use the calcium and magnesium for health, shells and colour, and enjoy the steady, buffered pH that comes with it. See what pH guppies need.
Fish that prefer soft water
Some species come from soft, acidic blackwater and colour up best there: many wild-type tetras and rasboras, bettas, apistogramma, discus and caridina shrimp. They can often adapt to moderately hard water, but breeding and peak colour usually need it softer. See softening water if you keep these.
Work with your water
Test your tap water’s GH and KH with a liquid test kit, then stock accordingly. It’s far easier and healthier than chemically softening every water change. Understand the numbers in KH and GH explained and check is tap water safe. More in the water testing hub.