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There is a reason fish die and it would help you more to investigate that rather than looking for "hardy" fish because no fish is hardy enough to withstand certain issues. You mention parameters being "almost ideal," but exactly what are the numbers?
Parameters technically refers to GH (general hardness), KH (carbonate hardness), pH and temperature. Can you give us the GH and pH of your source (tap) water?
Ammonia, nitrite and nitrate are not parameters but water conditions, and if you tested these it might help to give us those numbers too.
Aside from that, a Betta in with neon tetras is almost certain to end badly. I have seen Bettas easily eat neons, and neons (and many other small fish) will readily become fin nipping when placed in with a Betta. Even if you didn't see physical aggression, the Betta may well have understood the danger. Gourami is not quite the same, but the water numbers may tell us something.
Was the tank cycled before fish were introduced? That is another possible cause.
i'm not sure how to test my GH but being in the UK, apparently the water here is quite hard
I've ordered a new hang on back filter to switch out my current sponge filter which may be a bit too small for the tank. Could this help?
No, this will not help. A sponge filter is ideal in a 20g tank. I use this in all my tanks up to 40 gallons. The issue with dying fish has nothing to do with filtration if you had/have a sponge filter running.
Your nitrate at 40 ppm is very high for fish. Have you tested the tap water on its own for nitrate? You need to pin down if this nitrate is in (all or partly) the tap water, or occurring from the biological processes in the tank alone. Testing the tap water will tell you/us this.
When we know the GH/KH/pH of the tap water as essjay mentioned we can move ahead.
GH: 292.5ppm pH: 7.4 Nitrate: 80ppm
I couldn't find the KH
The KH is less of an issue here. But are these numbers for the tap water on its own, or the tank water? The nbitrate is now 80 ppm when it was 40 ppm previously. And the GH is high meaning fairly hard water.
I have had very good luck with scissor tails and tin foil barbs in pairs.So far I’ve had a Betta and 2 honey gouramis and they didn’t rlly last long even though my parameters are almost ideal.
I got a 20 gallon planted tank with 6 neon tetras and 5 amano shrimp.