Explain spiralling?
Was it spinning through the water, doing loop de loop, what?
Spinning or spiralling through the water is an indication of a bacterial or protozoan infection in the brain. This is usually caused by a dirty environment and lack of water changes.
The fish also appears black/ dark around the belly and that is normally caused by an internal bacterial infection.
If other fish ate the same veges and didn't die, then the veges were fine.
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If a fish ever dies, do the following.
Wipe the inside of the glass down with a clean fish sponge.
Do a 75% water change and gravel clean the substrate every day for a week. The water changes and gravel cleaning will reduce the number of disease organisms in the water and provide a cleaner environment for the fish to recover in.
Make sure any new water is free of chlorine/ chloramine before it is added to the tank.
Clean the filter if it hasn't been done in the last 2 weeks. However, if the filter is less than 6 weeks old, do not clean it. Wash the filter materials/ media in a bucket of tank water and re-use them. Tip the bucket of dirty water on the garden/ lawn. Cleaning the filter means less gunk and cleaner water with fewer pathogens.
Increase surface turbulence/ aeration to maximise the dissolved oxygen in the water.
Don't add any new fish for at least one month.