There are a few species of crabs that will be fine with a reef, but it's usually better to pull them out. Because their larvae will not survive in the tank, there's not much worry of them spreading.
Your salt mix should be buffered to get the right pH. There could be several problems:
-organic acids release by rotting material are neutralizing the carbonate in the water
-you have high CO2 from too little circulation or too much CO2 in the room air, which is becoming carbonic acid
-your salt mix is not well buffered.
If you take a sample of water from the tank and aerate it strongly for half an hour or so, and the pH goes up, you have CO2 problems. That can be helped by boosting the circulation.
If your alkalinity measures low (got a test kit yet?), then the buffer is being run down by acid. If your salt mix is decent, then a few heavy water changes should bring things back in line.
Your salt mix should be buffered to get the right pH. There could be several problems:
-organic acids release by rotting material are neutralizing the carbonate in the water
-you have high CO2 from too little circulation or too much CO2 in the room air, which is becoming carbonic acid
-your salt mix is not well buffered.
If you take a sample of water from the tank and aerate it strongly for half an hour or so, and the pH goes up, you have CO2 problems. That can be helped by boosting the circulation.
If your alkalinity measures low (got a test kit yet?), then the buffer is being run down by acid. If your salt mix is decent, then a few heavy water changes should bring things back in line.