Lemon juice can drop your pH which can kill your fish. If you aren't finding bodies, the mantis is being well fed. Remove the mantis - take all your rocks out and set them in bins - when the mantis is feeling a bit dry he'll drop out of the rocks into the water pooling in the bins... then you can do with it as you please - some hobbyists keep mantis tanks so that's a humane disposal for him if you can find a suitable home for him.
Kalkwasser paste or Joe's Juice will chemically kill aiptasia, or you can buy 2 or 4
peppermint shrimp and they'll clean those up for you naturally.
Before you add any livestock, make sure your parameters are stable.
HTH
Jenn
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Member of the "J" Crowd & the BRW Crowd!
LFS Owner:
Imagine Ocean

Just keep skimming, just keep skimming, just keep skimming, skimming skimming! What do we do? We skim, skim, skim!