I'm not sure what you mean by spotted dwarf puffer. Do you have a scientific name? In any case, it is not a fish you want to keep with GBR's, or anything for that matter. It is best in a species tank. Here is a site with info on true dwarf puffers:
http://dwarfpuffers.com/
If you are thinking of a juvenile green spotted puffer, they get much larger, and are brackish.
GBR's are not as delicate as they used to be. Most are tank bred nowadays. The actual pH is not as big a concern as keeping it stable. I think that your pH swing is fine.
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Eheim Pimp #254, Eheim Wolverine #1
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