Your gH, alkalinity and pH wonât all be near marine conditions since your salinity is only hovering at 1.005/1.006sg. Natural seawater alkalinity is about 125ppm/7dKh/2.5meqL, so 40ppm sounds about right for your current salinity.
That being said, alkalinity is rather important soley for pH stability. More important than GH. Your pH shouldnât have swings as large as youâve stated. I wouldnât go changing parameters in the tank now, but I would monitor parameters closely. Also note that any test kits designed for freshwater are probably not working accurately at 1.005/6sg. I couldnât decipher in your last post if that was the case or not.
When you do your first water change you could
add some kalkwasser or sodium bicarbonate to the mix, both are very effective and cheap ways to raise alkalinity. Adding a buffering substrate to the system might help some too. Youâre using RO water and adjusting parameters, so sticking to the same method and measurements once you dial in your parameters is crucial. As you increase salinity as the Columbian sharkcats age, you will need to re-evaluate as needed, most likely eschewing anything but the salt mix once you get towards half strength seawater.