My guess would be bacterial bloom. Beneficial bacterial are multiplying in large numbers, which clouds the water temporarily.
- Be careful of oxygen levels crashing in the tank. The bacteria consume it, and might begin to suffocate the fish. (They may hover at the top gasping if this is the case). A bubbler is a good stopgap.
- If and when you are able to get your own test numbers, you can make sure the "cycle" in the tank isn't becoming harmful to the fish. Though the growth of different bacteria: Ammonia (bad) becomes Nitrite (worse), which becomes Nitrate (relatively safe at normal levels, removed by plants or water changes). Since the water is consistently cloudy, it seems the bacterial are trying to catch up somewhere.
- If there is any possibility of extra food or waste (or worse, dead fish) being trapped in the gravel or under ornaments, it may be a good time for cleaning/vacuuming. Disturbing the gravel too much may upset the biological filtration, but it can't just be left in filth either. Undergravel filters can be a a bit of a maintenance challenge in general.