Fantail keeps laying on the bottom of the tank, always in the same spot.
Black moor keeps doing forward rolls and occasionally swims upside down. She never does it when I'm close to the tank but I'll see her do it from across the room. Recently found her laying upside down on the silicone anemone at the bottom of the tank.

I'm assuming it's a swim bladder issue. I've tried treating them by feeding peas, fasting, and with interpet anti internal bacteria. I thought their abdomens seemed a little swollen but now I'm not sure.

I've tested the water quality and the only issue I've found is that water hardness is higher than preferred (>14 but <21).

Any advice you could give me would be greatly appreciated.

I don't think the water hardness is of any concern.
What is much more important, though, are:
* the *exact* readings for ammonia/ammonium, nitrite and nitrates, plus details of the test kit you're using (as there's some variation in them and results are best interpreted in that context);
* tap water readings for the above;
* how many fish there are altogether, and size (dimensions and volume) of tank, and water maintenance regime;
* anything else that is added besides water dechlorinator?
These are crucial for cases such as these and, once we have these details, we can advise better.

Thank you for your response!

They are the only two fish I have. The tank is 200 litres, 100x40x50, with a sand substrate that's probably 3cm at the deepest mound and 0.5cm at the shallowest.

I try to aggravate the sand whenever I change the water. There are a few smatterings of gravel too. I kept it in a small pot in the tank when I switched from gravel to sand to help bacteria levels and the fish kept playing with it so I left a few areas in there.

I do a 25% water change weekly and use Interpet Bioactive tapsafe plus. I also add Love Fish Filter Boost. I do regular spot cleans with a large pipette.

I have been using Ciano water test strips. The readings are:
NO3: 25
NO2: 0
GH: 14-21
Cl2: 0
pH: 7.6
KH: 10

For ammonia, I have been using API test strips which give a reading of zero.

I was previously feeding them Aquarian Coldwater flake food and floating pond sticks but I switched to Tetra Goldfish Gold Japan because I thought maybe the floating sticks and dried flakes were causing these problems. I fasted them for 3 days last week and saw some improvements in the black moor but that's when the fantail seemed to get worse. I went back to regular feedings but when the black moor got worse and the fantail showed no improvement, I decided to fast them again because I'm seriously lost. I've already tried interpet anti internal bacteria and I didn't want to overmedicate.

I rescued these fish from my last boss who was going to flush them because he didn't want them in the office anymore. I'm a complete fish novice and I don't want to have saved them from a terrible fate just to subject them to some other horror.

Hmmm. There's nothing untoward leaping out at me (other than your last boss - glad he's in the past if that's his view of goldfish!). However, test strips tend to be very unreliable, which is why liquid-based ones are usually recommended. [Personally, I found the API ammonia test strips very difficult to read too, and the test strips particularly unreliable for PH/GH/KH.] I would get liquid-based ones for ammonia, nitrite and nitrate at least.

In the interim, I'd do the following to see if that helps:
* do more frequent and larger water changes such as 2-3 x 30% water changes per week;
* pre-soak the food in a small container of tank water before pouring it in;
* feed smaller portions more frequently;
* a one-off dose of aquarium salt (which will naturally dilute with subsequent water changes);
* check out https://injaf.org/aquarium-fish/the-go ... bladderbuoyancy-problems/ to see if there's anything else you haven't tried.

Thank you, I'll be sure to follow your advice! Fingers crossed it all works out. I'll definitely be getting some better test kits too.
Much appreciated