Okay so 5 fish overall in a 290 litre tank and from what I understand they're all single tailed goldfish right?
Your tank is massively over stocked with these fish, which means water quality may fluctuate and could possibly have been the cause of the issues with Dory.
Also with regards for water test kits, if you can get a liquid one I would recommend getting one. They work out more economical as you can get more tests out of them than you can using strips, so it would save money in the long run
I've had my liquid tests for over a year and still have tests left
I'm really not sure what to suggest for the long run, you have a few options, you can re-home some of the bigger fish in Spring, to someone who has a pond, generally a size of 5 inches is best to move a fish from a tank to a pond. So Waffles, Flounder and possibly Dory (depending on her size) could be re-homed. Sadie and Sally would be okay again, until they are a bit bigger.
Another option would be to consider having a pond yourself or buying a larger tank, but in all honesty a pond would be the cheaper option for you as large tanks carry a hefty price tag.
You're tank is ideal for fancy goldfish, but isn't big enough for the single tailed varieties permanently, not only because of the size they grow to! They can reach up to 30 cm's and need adequate space for exercise
But also because the fish are very messy and in small spaces their waste can build up and cause fish in the tank to become un well (as you've seen with Dory).
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40 gallons (48 US gallons, 180 litres) for the first fish with an extra 12 gallons (14 US gallons, 55 litres) for each extra fish. The tank needs to have a minimum length of 4 feet (120cm) to allow swimming room. Single tail goldfish are more suited to a pond when they are near full size to give them swimming room.
This chart shows how big single tailed goldfish should be at 2 years
So your tank would only be suitable for one or two single tailed goldfish while their young and those fish would need upgrading, not 5 permanently I'm afraid.
Now I can appreciate this a lot of information to take on board, so please take your time to consider what you'd like to do, as it's clear you care a lot about your fish and you have taken good care of them
Myself, I can appreciate the situation you're in, I have a single tailed goldfish in a 100 litre tank, she is stunted from living in a smaller tank before, but I intend to upgrade her to a pond as soon as she reaches around 5 or 6 inches. Whether that pond home will be with me I'm not sure, but I might have to re-home her.