Hello, I have just recently acquired a pair of green sunfish for a 75 gallon aquarium that has been cycled over the course of several weeks. Before I dive deeper into the condition, my readings are as follows (following Tetra EasyStrips & API Master Freshwater Test Kit); Ammonia - 0, Nitrite - 0, Nitrate - 8, pH - 7.4, Chlorine - 0, Alkalinity - 80/Moderate to Low, GH - 490/Very High. I've noticed that within the pair, the dominant male displays occasional flashing on the substrate, which is pea gravel, and rarely onto the hardscape and flora. This behavior was first noticeable a few days after acclimation, and dissipated in the following 2-3 days. Today, I did a 30% reduction of the tank, and filled it back up after adding Prime, both before and after (I'm very cautious about chlorinated water, so I double dose prior after post-water addition). About 3-4 hours after the water was added, the original sunfish began exhibiting the same 'symptom' that was portrayed originally after introduction, while the second sunfish hasn't noted any similar behavior. I began looking into skin & gill flukes, as well as gill-present ich, and couldn't find any information relating to this case; the only symptom he exhibits being flashing against edges in the tank (no bloody-red gills, no white spots, no labored breathing, no lethargic behavior, no feeding abstinence). Like mentioned before, the water is very hard, and that leads me to believe it may be due to the water hardness being reintroduced through a water change that is leading him to display such behavior. I have held off dosing with PraziPro or General Cure incase it isn't a parasitic plight for fear of stressing out the fish with its treatment. I'd like to know if anyone else could help identify this issue, or if there are similar instances relating to GH. Thanks in advance. Please let me know if there is any other information you would like provided.