90 gal with a snapper?

hi everyone im new here and to turtles. im a big into fish (cichlids) and am wanting to care for something new. so i got thinking about turtles. well i have been doing some research on some turtles but i want to keep my options open. im looking for a carnivore turtle bc i have plenty of baby fish that come from my other tanks. so i have a established 90 gal fish tank that im wanting it to go turtle. so can any of you make some suggestions on what turtle or turtles that can live comfortably in a 90 gal tank at adult size. im not wanting to have to upgrade to a larger tank so they need to be able to stay in the 90 for its lifespan. so let me know.
Thanks
Heather Andrews
Florida

Wanting it to stay in the tank for it's entire lifespan rules out a snapper.

If one assumes a "standard" ninety gallon, you're talking about a tank that's forty eight by eighteen as the base dimensions. That can only house a relatively small turtle long term, something around or under one foot in shell length.

If you have aquarium experience there are some facets that will transfer over very very well and some facets that are dramatically different when looking at keeping a turtle.

Foremost among these is the general approach to filtration. The amount of waste produced by fish based on their mass is substantially less than that produced by turtles; it's possible and desireable to aim at a biological filter bed in an aquarium, it's difficult to the point of impossibility to do that with most turtles and most enclosures. They simply produce more waste than can be handled by the bacteria load in a tank with a reasonable filtration system and everything will go fairly toxic fairly quickly if you're depending on a nitrogen cycle to keep things clean. With a turtle, you'll rely on heavy mechanical and chemical filtration methods and large frequent water changes. Depending on the species, you'd probably want to lose any substrate you have in the tank, as it becomes more of a detriment, trapping waste and preventing it from being collected in the filtration system, than a benefit. Live plants and any mix of species are also right out, as most turtles will take pieces out of anything remotely edible.

In a ninety, for some of the more commonly found species in pet shops... I'd look into mud and musk turtles if you really want a proportionally large enclosure or multiple animals. Red eared sliders, map and false map turtles, pond terrapins and sidenecks. Asian box turtles are generally more aquatic than the north american species sharing the common name and make an excellent choice as well, although they require a larger terrestrial area.

okay maybe not its WHOLE lifespan which ive read can be between 30 to 40 yrs. so is there still a possibility of having a snapper in my size tank (which is the normal 90). i did plan on taking out the substrat and maybe placing a few rocks on the bottom. definitly getting rid of the plants, and keeping some of the large pieces of petrified wood that i have (australian pine) in there. i do water changes weekly along with siphoning on my fish tanks but i did realize that the waste from a turtle is going to be much greater then my fish. so keeping the tank clean shouldn't be a problem i enjoy do the work. i have a emperor 400 and a emperor 280 on it for filtration, would that be enough? i was interested in the musks but by far the snappers are my favorite.