In wild breeding ponds, I found that there were no detectable levels of even nitrates, so it may be important for long-term health to keep the water very clean, even though they will tolerate higher nitrate levels in the water. The water in these ponds was trickle-fed from the water table, but was essentially still and in my experience, they don't like much water flow. Their body shape and swimming ability are not adapted to high-flow environments!
I housed 7 adults/subadults in a heavily planted four-foot aquarium with an Eheim canister filter hooked up to a rain-bar. The rain par was aimed so the jets hit the glass above the water and trickled down. This way, the tank was well filtered (the canister filter has a large capacity for filter media and quite high through-put), but here was practically no current at all.
A sponge filter driven by an air pump would also work, but you will need to make sure they are supplemented with enough water changes.
You could also use an in-tank box filter and either have the output flow broken up with rocks or the tank glass, or attach it to a rain bar (you can DIY one of these with the correct gauge of aquarium tubing with the end blocked up and holes drilled along the length. Make sure you drill the holes - simply poking them won't work as the rubber tends to close up and small bits of debris clog up the tiny remaining holes).
I would also suggest having plenty of vegetation in the water so they can sit with their heads out (mine were usually in this position when not feeding or breeding). A cork-bark island is also a good idea, in my experience. One other thing is to make sure that the tank is well ventilated, so the air is not too stagnant above the water line. I had an issue with a skin infection with mine until I increased ventilation; Tylototriton seem quite susceptible to skin infections and allowing them to dry their skin off a bit by coming out the water can allow them to deal with them themselves if there is a problem.
Hope this helps
C
