I found some interesting items concerning clawed frogs and ammonia:

Xenopus can tolerate much higher ammonia levels than fish. High ammonia levels in the water is very stressful and inhibit ammonia excretion which causes the frog to change to ureotelic excretion. Ammonia levels should be maintained below 0.50 mg/L and NH3 below 0.02 mg/L.

Also, water temperature has a profound effect on water quality. Dissolved oxygen levels decrease as water temperature increases. Also, unionized ammonia (very toxic to Xenopus) levels increase as water temperature increases.

It is ideal to keep Xenopus at a water temperature of 21(C) or 70(F) with a pH of 7.0. Temperatures exceeding 30(C) or 86(F) are lethal. In fact, keeping temperatures above 24(C) or 75(F) will make it difficult to keep unionized ammonia (NH3) below optimal levels.