Nitrates will not go down.

[Lasse]

This i understand. They can store waste which is then converted to nitrate, is what i should have said.
Sort of like the old saying "canisters are nitrate factories"

In that case - the aquarium is cycled - the OP report zero NH3/NH4 and zero NO2 but high NO3. That´s a possibility but the OP also report huge WC without any downgoing in the NO3 level. A well working nitrification cycle is fast but not fast as that. I can only see some plausible reasons to the high readings - based on what the OP have written. The OP also write tha the sand was cleaned at least in a light way

I've sifted the sand to take out all the empty shells, left them in water for half an hour and tested the water and it was clean.

1 Faulty NO3 test
2 Faulty NO2 test (NO2 interfere with NO3 tests) Scroll down to english here this brand interfere with a factor of 50 - some brands have the factor 100
3 The water contain other ions that interfere with the type of analyse the OP using
4 The initial NO3 concentration was sky high - 500 - 1000 ppm

@Tainum 40 - 60 ppm NO3 is not any problems for fish and most invertebrates, If you confirm your measurements to be around 40 - 60 ppm you can start your system slowly and during the start up do more WC than normal and slowly introduce some other type of NO3 management.

A way of testing your analyse and/or see if it the NO3 concentration is higher than the tests specifications is to mix a new batch of salt water. test the new batch - if zero - make a dilution 1 parts old saltwater and 1 part new saltwater. Multiply the result with 2. Still not a good reading - take it 1:2 (multiply the result with 3) - still not good make it 1:3 and further on (multiply the result with x+y). I prefer to use fresh mixed saltwater for dilution instead of RO water because with sea water (NO3=0) you get all of the disturbing ions with.

Sincerely Lasse