Fish or Bookshelves?

I am at a crossroads of fish and books.

My 75 gallon has given me a lot of trouble over the years--failure to manage live plants, long battles with algae, and most notably, a murder mystery to baffle even Nancy Drew.

Something was wrong with the water. I knew that, because the last angelfish standing (swimming) was severely stunted. But, curiously, the water parameters all turned out fine no matter how many tests I threw at it except for an overall hardness that rocked the charts. Not just that, but the fish didn't die in the order they should have: if it's a disease or poor water quality, the most delicate die first, if it's a bully, the weeniest die first. There was no rhyme or reason. Once a month, like clockwork, one fish would die. Maybe it was a ram (delicate, invites bullying), maybe it was a cory (hardy, ignored by tankmates). For the past half year it settled into equilibrium with one angelfish and three cories. In a vast 75 gallon.

The angelfish died last week, which leaves me with a question.

Take the tank apart, put the cories into the 20 gallon with one of my bettas, and put up book shelves? I do need book shelves, I'm running out of space for books and knick-knacks.

Or... try again with very different kinds of fish and plastic plants? I know that my ability to care for fish is fine, given that the only aquarium that's ever given me trouble has been the 75 gallon, which is why I am not quite ready to throw in the towel yet.

For a while I'd been thinking about trying Malawi cichlids since they thrive in hard water, but a cichlid tank would be pretty expensive to start out. If I try again it'll probably be mostly livebearers. With plastic plants.

I'm doing good with live plants in my betta tanks, but that's because I can actually reach the bottom to do maintenance. ...And they survive with the ambient sunlight in my living room orz so much light so much algae. But in the 75? Eff live plants. Eff them.