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Could be a combination of the salt (sand if recently added) as well as the low GPH
in the tank. The whisper 60 is only 270 GPH
rating when empty. Once it has media, the flow will be reduced a fair amount. Do you have additional PH
in the tank as well?
Good water rotation starts at 6x rotation per hour and that is considered on the low side. Ideally you will have to determine the total GPH
based on the type of tank you will end up having but for the most part you should try bringing that up to 400+ GPH
The low ph
could again be from the salt but low GPH
and possibley having the tank covered would affect it as well.
Any additional info you could provide about the set up would help.