Use a toilet tank and bowl combination the manufacturer specifically approves. Matching color, bolt spacing or brand does not establish compatible flushing performance or a sound seal. Check both model numbers against the combination specification and repair-parts documentation. If the approved replacement is unavailable, replacing the complete toilet may be the appropriate choice.
Check the combination, not just the brand
Find the tank and bowl model numbers and consult the manufacturer's specification or parts diagram. The combination's product number can be different from either component number. That is expected, not evidence that the pieces are mismatched.
For example, Kohler's cited K-6355 Persuade Curv specification lists a K-4326 bowl and K-3569 tank as its included components. The cited K-3999 Highline sheet lists a K-4467 tank with a K-4199 bowl. These are examples of specified combinations, not permission to exchange the tanks between them or assume all Kohler two-piece toilets share parts.
Ask the manufacturer to confirm any proposed substitute using both model numbers. A discontinued listing or an online seller's claim that a tank is universal does not replace that confirmation.
Physical fit is only one requirement
Bolt positions and outlet openings matter, but so do the mating gasket, tank support and flushing design. A gravity tank is not a substitute for a pressure-assisted or flushometer system simply because the outside china looks similar. The water-use rating and performance information describe the specified fixture, not an improvised combination.
Do not enlarge porcelain holes, grind mounting surfaces or make a stack of improvised seals to force a fit. Installation hardware must follow the approved model instructions. Tightening harder to stop a leak can damage the fixture rather than solve a compatibility problem.
Decide whether a component replacement is practical
An approved tank-only replacement can preserve a sound bowl. A cracked bowl, unavailable approved tank or incompatible flushing assembly can make a complete replacement the better route. Knowing when to replace a toilet separates fixture failure from repairable internal parts.
If the issue is inside an otherwise compatible tank, replacing a toilet flush valve may be the relevant repair. If the whole toilet must go, choosing a toilet for your home covers rough-in, bowl shape and clearances. Confirm the component match before arranging installation, not after the old tank has been removed.