Fresh water feels like a new river
To you, a water change restores the same tank. To your slider, everything about its world has shifted at once: the smell of the water, its temperature, the current from the refilled filter, even the view through suddenly crystal-clear glass. Sliders investigate change the physical way — by trying to climb out and explore whatever this new place is. Fresh tap water can also genuinely irritate: the chlorine and chloramine added to drinking water are harmless at your tap but rough on a turtle's eyes and skin, and a turtle paddling frantically at the walls in the hour after a change may be trying to climb out of water that stings. A temperature gap between the new water and the old produces the same restlessness, because the turtle's whole body chemistry runs at the water's temperature.
The usual suspects, in order
Start with temperature: if the replacement water went in noticeably cooler or warmer than the tank, expect climbing until things equalise. Next, the chemical sting — untreated tap water is the most common reason for climbing that begins immediately after a change and fades over the following hours as the chemicals gas off or the turtle adjusts. Then check what else your change disturbed: a basking platform refitted at a different angle or height, a filter outlet now blasting a stronger current, or a water level that has risen close enough to the rim to make escape look genuinely possible. Finally, some climbing after a change is simple high spirits — a clean tank, clear sightlines and a watchful owner nearby are stimulation, and a young slider will test the walls of a refreshed world before settling back to basking.
Water changes that do not trigger the climb
Match the new water to the tank before it goes in: let it stand in a bucket until it reaches the same temperature, and treat it to remove chlorine and chloramine whenever your supply needs it. Partial changes unsettle a slider far less than emptying the tank, so changing a portion of the water more often usually works better than a rare full strip-down — and it keeps the filter's bacteria colony intact as well. Put every fitting back exactly as it was: basking platform at the same height, filter outlet aimed the same way, water filled to the same line. A lid or a rim the turtle cannot hook a claw over matters more on change day than any other, because that is when escape attempts peak. Most sliders settle within an hour or two of a gentle change done this way.
Climbing that means trouble instead
Climbing that continues long after the water has settled, or that comes with company, needs a different reading. A turtle that climbs and gasps, holds its mouth open, blows bubbles, or swims lopsided and circles to one side may be struggling to breathe, and a reptile vet should see it promptly. A female slider that climbs frantically day after day, digs with her hind legs and refuses food may be carrying eggs and searching for land to lay them in — she needs a proper nesting area, and a vet's help if the searching continues without laying. For a related question, put a follow-up question to KinsLe's Kimmy; she answers using the KinsLe Answers library.