Does this dishwasher need a commercial descaler?

[ImpatientGriselda2]

For a few months it has often left a wodge of stuck-together powder detergent (Finish) in the dispenser, (which gets reused for a subsequent wash) and lately the items are worse rinsed than they used to be. In the last couple of weeks cutlery and the rims of glasses and mugs often need to be rinsed under the tap to get visible detergent remains off. This is regardless of the rinse aid and salt being topped up.

About 3 weeks ago I put a lot of vinegar in the base to descale it; this might have improved things slightly for a few days, but not longer term.
Reluctant to use aggressive commercial descalers but perhaps it's necessary here, will just run 2 or 3 washes to get it out before using any items that go directly in the mouth. It's a hard water area and the machine is a Zanussi. Which descalers would you recommend? Thanks.

I either buy the Dr Beckmann 'Service It' one from the supermarket or a cheap one from Amazon. I've used this one and it was fine. They're all just citric acid when all said and done.

Are you putting salt in? Dw shouldn't need de scaling because they have inbuilt water softeners.

Yes. Salt, rinse aid. All Finish. The water here is the hardest I've ever seen though.

The detergent dispenser opens, and the dishes are mostly clean, it's just that there's this lump of caked powder detergent left, and the thing is rinsing inadequately. Already tried using slightly less detergent, but there was still most of it left, caked. I just read something else suggesting the spray arm might be blocked so perhaps that needs checked out. Or maybe it's something else again.

You can put the powder on the bottom of the dw, I don't bother with the dispenser.

You can take the rota arms off and flush them out with hot soapy water.

if you open the door mid-cycle, it is fully hot?

Have you taken out and cleaned all the filters, and the arms?

A dwr tablet left in the sink will dissolve in very hot water without even needing to be stirred (try it), so there is something wrong.

p.s.

your machine's softener can be adjusted according to the hardness of the water, check the instructions for how.