@Mieke7 yo mate, i am new to here, i hope you have already sort out the wate quality.
What i suggest you is that removing the ornaments stuff from your tank, i dont think these stuff would need it, normally goldfish stay in a 'air tank' which mean only water. If you do like to put them in, you can put it back once your water clear.
Washing your filter media, change the water 1/2 starting build the nitrifying bacteria properly. remember no nitrifying bacteria = no clear water.
Hope it can helps?