Help with 26 mounting

[Zos4me]

I recently purchased a hydra 26 fromantic my lfs and I bought the hms arm to go with it.I get it all together and it won't fit tight to the tank .I have a standard 30 gallon long with rim. Can anyone give me some Insight I'm pulling my hair out after spending 425 dollars on this thing I'm besides myself
Thanks in advance

 

[Ranjib]

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