Channel catfish (Ictalurus punctatus) is one of the most important farmed fish in the United States. Among various water quality parameters affecting the catfish survival and performance, increase in water borne hardness is one of the major concerns. Despite various reports on the effect of water hardness on growth performance of commercial important fish; the maximum tolerance range of channel catfish to elevated hardness is also not yet reported. Additionally, the mitigating effects of elevated water hardness on channel catfish in relation to increased environmental ammonia and increased salinity have also not been extensively reported on. To provide primary information on the tolerance of this species to elevated hardness, a 96 h-LC50 toxicity test was conducted. Following a range finding test, the 96 h-LC50 value was found to be 4940 mg/L (C.I. 4970-5223 mg/L). This maximum tolerance value in catfish is many times higher than other commercially important fish including salmonids, largemouth bass, striped bass. To get a better insight in determining the reasons for catfish's ability to tolerate a very high hardness and the possible interactions with other water quality parameters, a microscopic examination into the tissue (gills, liver, intestine and skin) morphological modification (remodeling) was performed in function of step-wise increment in water hardness (ranging from 200 mg up to 4000 mg/L) following a 15 days trial and at the three different levels [Control (100 mg/L), 494 mg/L, and 1250 mg/L] following a 60 day growth trial and 21 day challenge trial. Fish were exposed to 10 and 25% of the 96h-LC50 for two months, following the two months they were exposed to elevated environmental ammonia and elevated salinity for 21 days. Following both the two months and 21-day trials, tissue, water, and blood samples were taken for analysis. Various growth parameters were calculated following the two-month trial. It was found that there were differences in the growth parameters between the three dosage groups. There were also differences in the plasma ammonia levels, ammonia and urea excretion rates, oxygen consumption, and calcium concentrations within the various tissue and blood samples from the two trials.
At relatively lower concentration (200-1000 mg/L), primary and secondary lamellae appeared normal. Remarkable alterations were evident at relatively elevated hardness 1500 - 3000 mg/L wherein interlamellar cell mass (ILCM) were momentously developed. This reorganization is an attempt to reduce the surface area presumably protecting against the water borne hardness. Likewise, at higher hardness, liver, and intestine displayed compensatory adjustment, enabling catfish to tolerate high level of hardness.