Well treatment methods and fluids

A method includes forming a well treatment fluid by combining an aqueous fluid, a viscosifying agent, a crosslinker, and a rheology modifier containing an aldehyde. The viscosifying agent is crosslinked, increasing viscosity of the fluid. The method includes treating a well with the fluid exhibiting the increased viscosity, chemically changing the aldehyde after the crosslinking and thereby forming an acid, and decreasing viscosity of the crosslinked fluid as a result of forming the acid. The aldehyde may be glyoxal which forms oxalic acid, breaking a crosslink. A well treatment fluid is formulated with ingredients including an aqueous fluid and a viscosifying agent and a crosslinker configured to crosslink the viscosifying agent. The fluid includes a rheology modifier containing an aldehyde. The fluid may lack an aldehyde crosslinking delay additive.