[AquaticLife] Replacing internal filter
It looks like you have too large of a power head. 1500 L/H is almost 20X your tank size. You should look for a power head around 500 L/H at most.
You should not be turning your filter off at night. If you leave it off for 10 hours, ALL of the nitrifying bacteria will die.. or maybe very few might live. You are basically killing off your biological filtration every time you do that.
At the least, leave the internal filter on 24 hours a day so at least it stays fully cycled at all times. The N-bacteria need a constant source of oxygenated water and ammonia/nitrite and trace elements to stay alive. When you turn off the filter, the N-bacteria start dying off due to a lack of those essential items.
Until you find a smaller power head, you can either run the current one or not… if you don’t, whenever you do a PWC, you should vacuum your gravel well where ever you do not have any plants.
Actually… with all the filtration you now have… you really need to work on getting a bigger tank! 
Lenny Vasbinder Fish Blog - http://GoldLenny.blogspot.com
Category: Philippines Internet Koi Society
