Forgive me if this is a stupid question, but is Acera a 9-speed system? Do you have 9-speed shifters? You can't just stick a 9-speed cassette on with 7 or 8 speed shifters. Well, you can, I guess, if you have an 8-9 speed freehub body, but not if you have a 7sp. And the indexing won't work though you might accidentally hit right on a few cogs. I only ask because you said you had a 7sp chain. If I have misunderstood, please disregard all this.
If you do have a 9sp system you definitely need a 9sp chain. A 7sp will be too wide for the narrower 9sp cog spacing. On the other hand, you can use a 9sp chain on a lower speed system. Sometimes they even work better. I like SRAM chains.
FWIW,
Raymond
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