I love fish they are very cute.
Possible fixes
comma and conjunction
Semicolon
Subordinating conjunction to make 1 clause dependent
I ran out of water I had to go to the store.
Fused sentence
I don't like mosquitos, nor cockroaches, nor wasps.
This sentence can be turned into a list by removing conjunctions
I don't like mosquitos, cockroaches, or wasps.
He ran and jumped and laughed for joy.
Polysyndeton
I need to get some sleep, I'm very tired.
Add a conjunction
for example, "I need to get some sleep, for I'm very tired."
Running to the store.
One of my dream fish is a snowflake eel they need large tanks and can't be kept with other fish.
Add a comma and conjunction
Example
One of my dream fish is a snowflake eel, but they need large tanks and can't be kept with other fish.
Usually, I take a walk every day while the sun sets, but it was raining today.
neither
I was hungry I went to lunch
conjunction and comma, semicolon, or split with periods
I went to my favorite restaurant, I love to eat seafood.
comma splice