I have a Biorb life 60l. Been set up since January. Water tested locally at reliable fish place and all in order. Tank came as a kit with pump and ceramic stuff. Have added only Biorb plants and. £50 feature. 50% water ch@ng3d with tap water treatment 10 days ago. Only have goldfish. 2 died in last week. 6 small and one about 2” left. All top fins flat. Feeding well and active but as fins down doesn’t look right. Changed filter and carbon 3 days ago washed in tank water. Any advice please. Bill

When was the water tested? I'd be extremely surprised if all is now as it should be as the tank is very overstocked and it's likely ammonia or nitrite have risen as a consequence, hence the deaths. Unfortunately, it isn't big enough for even one goldfish .... see this article for reasons why: https://www.fishkeeping.co.uk/articles ... -size-life-expectancy.htm

The best thing to do is return them to the shop and get something more suited to a small tank.....maybe White Cloud Mountain Minnows, though the tank being tall rather than long is not ideal for them either. If it was mine I'd add a heater and go for either a single male Betta splendens or a shoal of tiny tropicals such as mosquito rasbora or celestial pearl danios, depending on how hard your water is.

Following on from the very helpful advice above, a few questions/comments:
* Did you do a fishless cycle before getting the fish (ie a 6-8 week process of preparing the filter to build up sufficient beneficial bacteria be able to process ammonia, using an ammonia source to mimic fish waste https://www.fishkeeping.co.uk/articles ... hless-cycling-article.htm)?
If not, and you simply set up the tank and added tap water with water dechlorinator and switched on the filter, then got that water tested before adding fish, its readings will be "all in order" - but that will change as soon as fish are added and waste/ammonia produced. If this fishless cycle wasn't undertaken, now that you've got fish, ordinarily I'd advise trying to prevent the immediate deaths of the others by following this fish-in cycle guide https://www.fishkeeping.co.uk/articles ... ammonia-nitrite-spike.htm - *however*, given how unsuitable the tank size is for goldfish, I would follow Fishlady's advice and return/exchange them. It might be that they give you a credit note on their return and you can get fish at a later date, once you've done a fishless cycle.
* You mentioned "changing the filter" - if you mean you discarded the old filter cartridge and carbon, replacing it with new cartridge or other filter media and carbon, then this is going to result in prolonging/re-starting the cycle - you need to keep the same filter media and only squeeze/rinse it gently during the weekly water change in being-discarded tank water.
* The local fish place oughtn't to have sold you *any* goldfish for that size of tank and its limited 'footprint', so you are well within your rights to return those fish and get something more suitable for its size such as those suggested by Fishlady.
* You ought to buy a liquid-based testing kit (ammonia, nitrite, nitrates) - not a strip-based one - to monitor water quality in between water changes and ensure that water is kept consistently at safe levels.