Hello, Sun Kim.
My name is Michael, customer / product support Manager for Coral Vue.
I have read through the entire ticket trying to gather the issues you are having with this skimmer on your system.
While I see that Carlos has responded with some great information regarding the size of skimmer on your tank, it seems that you are not being satisfied with the responses thus far.
I appreciate your patients in this matter and would like to offer you a few things to check that are common things that can cause HOB skimmers to act up.
Even though we have established that the skimmer can and will be unreliable / erratic in foam production being it is oversized for the system volume I have some suggestions and comments I would like to address.
Things to Check
Start out by checking the venturi/ air silencer. Make sure that it is free and clear of any salt creep, dust trapped inside or a combo of both as this will cause less air to be taken in by the pump and the water level will rise in the skimmer body overflowing the cup. Less or no air will cause the pump to run at a higher volume of water pumped.
The air line from the air silencer to the venturi fitting on the pump, make sure that it is free from build up of salt creep and dust as this is an unseen issue that causes much frustration for many folks.
Both of these items can be cleaned with vinegar / water solution or with regular tap water if needed though hard deposits will need to be dissolved with vinegar.
Most of the time if bubble production becomes an issue over time with a great working skimmer the air intake system is in need of a good deep cleaning.
Just so I understand correctly what you are stating is that the bubble production is starting / stopping periodically correct ?? But the pump still remains running Right ??
If so this is why I am bringing up the air intake system to be cleaned. Salt creep and dust enters this part of the skimmer and over time with moisture will creating erratic air production, This can also explain why the skimmer will run fine then you shut it down for tank water change and it acts up after you power it back on because the material in the venturi may have been dislodged by the water rising in the tube some when off and gets stuck in the line keeping air out of the pump when powered back on. Same can happen by not doing anything just running the skimmer and debris falling loose in the line causing issues.
What Can be done is to take some Salt water from your tank or a bit of RODI water in a cup then either place the air silencer into the cup of water allowing it to pull the water through into the pump or by injecting a few milliliters into the silencer with a plastic syringe. If debris is the issue then you should notice a difference in bubble production after flushing it.
I would still recommend taking it apart and give a thorough cleaning to the air system and allowing it to dry then assembling the skimmer again so that dust does not start to stick to wet surfaces of the air silencer and tubing immediately
Again thank you for bearing with us on this issue, We look forward to your response and video later today.
Sincerely Michael