I know that there are listed ideal water parameters but what I am wondering is what is really dangerous to clams when it comes to water condition?which clam is especially sensitive to bad water conditions? Will high nitrates(namely 20 parts per million) kill a clam?
if your trying to lower nitrates the first thing i would do is increase flow to keep uneaten food up in the water column so it can be filtered out by your skimmer. clean all detritus from the tank and sump. skim on the wet side and do 10% water changes every week
I used to conduct a 15% water change every 4 months, use and underrated EuroReef Skimmer, and feed "excessively". I had zero nitrite, nitrate, ammonia, phosphates, and no problems with algae for several years. It was only after I started "defying" the rules of good aquarium husbandry that I had success, especially with clams. I find that the aquarist themself, more often than not, tend to be the culprit of aquarium issues. I would suggest leaving the aquarium alone for a month and let all of the parameters and microbial flora equilibrate. Doing so will allow you to more accurately examine the aquarium, as from my experience the changes in aquariums occur on the scale of several weeks (not days or a single week). By constantly trying to find problems, you are not allowing your tank to adjust to the situation on its own. And in fact these adjustments are often maladjustments. I would just feed the fish until satiation (which shouldn't be much food with only three), keep doing routine water topoff, and examine calcium, alkalinity, and pH for ionic balance. After that just sit back and see if the problem solves itself!