Low Alk

I know how to correct this problem, but what causes it?

When I tested the alk last it was at 4.8 dkh That is way too low. I do a 30G water change every other week and I use Oceanic Salt.

The weird thing is that I am using the same salt for my sea horse tank and it tests just fine. I am dosing the baked baking soda so I will get it back on track, but I want to know why it is so low. This is NOT the first time I've had this problem. I finally get the Nitrates to 0 and alk becomes the new problem. Any ideas??

Thanks,
Angie

My guess would be that the calcium/alk levels in Oceanic salt are out of balance.From what i've heard Oceanic mixes up at 500ppm calcium,that level has a balanced alkalinity of around 18dKH.Whn calcium levels are supersaturated(I would consider 500 supersaturated),and if the PH/CO2 levels are right,then calcium can bond with carbonate/bicarbonate(alkalinity)thus lowering your levels.

Should I alternate my water changes with Instant ocean salt then? Instant ocean is lower in calcium and alk levels from what I've experienced. I switched from Instant ocean to Oceanic because I used to have calcium levels around 250-350 and I don't have sps. Is there a perfect salt?

I know Repto uses a mix of IO and Oceanic.I think it would be less stressful if you used equal amounts of the salt when mixing up water for waterchanges rather than just alternating.I've heard of to many people having problems when switching salts.