You wrote very factual based information. I don't think that you will have a problem in terms of being 'neutral'. There isn't a lot of room to be biased here. Some grammatical errors but only minors ones. My only concern is whether or not Wikipedia will have a problem in recognizing the paraphrasing. Aroojazaidi (talk) 15:26, 26 April 2017 (UTC)
Difference from goldfish:This paragraph is very informative and applies to this section. The first three sentences of the paragraph can be added to the second paragraph of that section. The fourth sentence could probably go right after the sentence ending "and can be difficult to tell apart from koi when immature." The last sentence in your paragraph would be better in the Goldfish page instead of the Koi page.
Koi:This section would be best put in the history section of the page. Although the page already talks about it being a hobby("The hobby of keeping koi eventually spread worldwide. Koi are now sold in many pet aquarium shops, with higher-quality fish available from specialist dealers.[5][6]") your paragraph would be better and more informative.
Disease: This sentence would be best put right before the sentence that talks about the two major health concerns for that section
Breeding: This paragraph could replace the entire first sentence of the section and could probably be its own paragraph in that section Beastly334 (talk) 15:45, 26 April 2017 (UTC)
The talk about diseases would be better followed by solutions. Just something to expand on. Dest Tiny (talk) 15:49, 26 April 2017 (UTC)Dest Tiny