Given how most zoos exhibit them, hippos are honestly what I'd consider the worst investment a zoo could make right now. Spend millions of dollars to build an exhibit that is barely adequate for a smaller-than-ideal social grouping, and then spend a ridiculous amount of money on life support systems and exhibit maintenance in order to keep the water crystal clear. Rhinos are a species that don't need much for an exhibit to be great, compared to hippos which require as much if not more space than elephants (due to the large water requirement). I'm sure that hippos bring in some new people (Fiona exists, after all), but all in all I doubt hippos are any more popular than rhinos for the average zoo visitor. As far as Houston is concerned, hippos also likely aren't a great investment seeing as at least three major nearby zoos (Dallas, Fort Worth, San Antonio) keep them.For the long term the hippos really would have been better. Not only do I believe they would have brought in more guests (hence more money) than rhinos, but in the long term it would have been easier to save up for a new rhino exhibit, plus the hippos would have better fit the African forest theme than rhinos.