Originally Posted by Gus-Gus

Your fish are dying because of your water conditions.
Firstly, you clean your filter every 2 weeks. You are killing/ removing the beneficial bacteria in the filter. The bacteria in the filter is what breaks down the nitrites and ammonia in the water (from fish waste) and breaking them down into nitrates which real plants in the tank would then use as food to grow. The fish die because the ammonia and nitrites build up in the water and poison the fish.
STOP WASHING YOUR FILTER. The filter material should be gunky and dirty looking - that is what the bacteria thrive on in order to create good water conditions for the fish to live. Only clean it once every 6 months if at all.
The same with the gravel - the gravel will over time accumulate gunk with again contains bacteria which the plants need to thrive in order to create good living conditions for the fish. STOP WASHING THE GRAVEL.
Second, remove the plastic plants and have real plants.
Thirdly, RO water is water with nothing in it - no minerals or trace elements that plants and fish need in order to live let alone thrive. STOP USING RO WATER. Use aged/ dechlorinated tap water instead.
Fourthly, using any sort of medication in your tank kills the bacteria in the filter - no matter what the manufacturer claims.
Finally, your tank is too small for so many fish. At most you should have two gouramis in a tank of that size.
I hope this helps.