Minecraft 1.20.5 has changed syntax on many commands, so most commands will no longer work without an update.
An update is in progress, you can check compatibility progress here.
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Try the Commands Troubleshooting and Help page if you get stuff with server errors.
Minecraft Pufferfish Fish NBT Data Tags
Using the summon command to spawn in a minecraft pufferfish is simple, take the command and enter into chat and presto you have a pufferfish.
Summon pufferfish that will not despawn
Fish placed with buckets do not despawn naturally, the {FromBucket:1} tag can be set to make them perminant as if they were from a bucket.
/summon minecraft:pufferfish ~ ~ ~ {FromBucket:1}Suggestions or found a bug
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If you're looking for a particular summon mob generator you can link to these specific versions of the command generator.