Goldfish and Fantail sitting on bottom not feeding - HELP

Szaki wrote:
PetsMart sells or can be bought on line, water testing kits. One I like, has
5 small pads on it, (Quick Dip 5-N-1 Test Strips) Made by Jungle, testing 5
different conditions, including Nitrites. You don't have to be rocket
scientist, just match the colors. Cost about $12, mine lasted all most 2
years, depends how frequently do the test.

Bit more expensive in the UK

But theres a local pet place that does free water testing
Took some today and the guy said that the ammonia was ok -slightly above 0

But said the nitrate or nitrite was too high
came up mid way on his scale instead of 0

He told me not to change the water because that would make it worse??
Just told me to leave it for a week and then change 20% of water and to
hoover all the old food and fish poo from the stones...!

Does that sound at all right?