You're probably talking about aiptasia, yes they are a problem.
Skip the kalk additive to the boiling water, I haven't seen it add much benefit. If you get them with the hot water it will be enough. But, you don't always "get" them. They are surprisingly fast and will retract far enough to avoid the jet of boiling water. So you have to get that one next round. There will be many rounds...
I agree that the substrate change will not cure the diatom issue. However, I say go for it while you can get a good deal on the sand, you will not regret it. I bought my tank as a complete system and it came with crushed coral. The switch-out was a big pain but was worth every bit of effort. Looks much nicer and all the inhabitants look more comfortable.
My tip: get every morsel of the old substrate you can. It will be surprisingly obvious after the switch.
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