The " Time Bomb " comes with any UGF Filter that does not circulate properly.
The whole premice to a UGF is that the water is constantly being "filtered" by the bacteria bed in the substrate.
Even with good water flow through the substrate(front to back of plate) you had to keep the fish mulm vaccumed or it would plug
areas in the plate. That build up you get is the Bomb I speak of. If you have ever taken down a tank that has an improperly working UGF you would SMELL
what I mean !
There is no bacteria in the world that can catch up to that mess.
So now with a planted tank you have the roots that will take care of such toxic substrate buildups BUT the roots cannot get there because they are bound up by the UGF plate. This can happen very quickly too.
Those 1/2" gaps under the plate now become sealed and very toxic just waiting to be released.
Now one day you decide to "uproot" some old plants and try something new...
Wellguess what ...
you have just released those evil fish killing GASES !
I have always used and LOVED UGF's but in the planted world they have no place I have learned.
The only disappointment in my 23 years of fishkeeping is that I didnt plant my tanks all along !