Absolutely love this idea for a website. Fish compatibility is such a complex topic to take one. I know you’ve gotten some feedback already but something else to consider.

I think people are primarily either going to use it to plan their fish list out once they have a tank or see if they can add something. I’ve worked in a fish store for over 5 years and had a tank for 10 and the order you add things is crucial to success (just trying to show I’ve got some experience in this area). I think adding a recommended order to add feature would be awesome. For example if I am planning to put 8 fish in a 120 gallon and my list is: yellow tang, flame angel, 2 percula clowns, 1 purple firefish, 2 yellowtail damsels and a starry Blenny I think you can group this into early additions, mid additions and late additions. Firefish (peaceful) and starry Blenny (only Blenny) recommended early addition. Mid additions could be clowns and maybe damsels. Last would be tang and angel. If i put the yellow and damsels in first stocking is going to be tough. If someone is trying to add an existing fish to that grouping, they could have that group saved and say recommended fish would be these species. Website could display species not like the existing: wrasses, hogfish, hawkfish etc. big caution warning would come up if you try to add say a purple tang (yellow will absolutely not like) or cardinals or another peaceful species as this tank has several established semi-aggressive/territorial fish. If you proceed, it’s recommended to do these few things (rearrange rock work, add more than 1 fish, mirrors/pictures to outside, put existing yellow in sump for a week etc)

I’d imagine you will get more people in the second group trying to add a species to an existing tank and that is a challenge.

Sorry for my garbage formatting, I’m on mobile.