I agree with T/N completely!
Very good info, could use some organization work, but overall i'm impressed - great job.
I know you love NLS but it's not appropriate to endorse it specifically in an article that is meant to be a general guide to goldfish - at least not when you state as fact that it is the best. Clarify that this is your opinion or take it out altogether.
A note on writing style. The article is really great, but it's a little wishy-washy. It's very passive and wordy, and as a result, it seems as though you're not sure about what you're saying. You use "in general", "it may seem", "it has been shown" where you really don't need them. The article is sooo long, and with a few changes it could be shortened without taking out any essential information. For example:
There are some species that have proven to be great tankmates for goldfish. These fish have compatible water parameter and temperature needs. They also have behaviors that make them very compatible with goldfish.
Could easily be made stronger with a little more active tone:
Good goldfish tankmates have compatible water parameter and temperature needs and get along well in a community setting.
Normally I wouldn't mention style, but this article has such great information, the passive voice really holds it back from having the impact it should. Haha, can you tell I've written a few too many essays and articles? You can leave it as it is if you like.
Anyway, I think you did a really fantastic job. Nice work!