He may have some kind of vision issues, have you tried the moonlight on his tank, maybe it is the type of light ( led ) or color spectrum, or kelvin temp. Try a 12/13 watt CF aim it from the side. Could he have been abused by a previous care taker?
A guy on youtube called Pondguru shows how to make simple, cheap bio filters, out of various plastic bottles, air pumps, and bio media. For your tank to be successful you will need to kick up it's bio filter.
Your fish and the TSS will work together at the same time... adding the TSS to the filter just gives the bio media a good boost. Since you don't have gravel, most of the surface for bb to grow is going to be in the filter.
I agree with el337 your fish is suffering from nitrite exposure, he is slowly suffocating. If you can move him to a shallow container, so he can get some air that may help. A temporary treatment of 1 TSP of AQ or kosher salt to one gallon of water has saved many fish from nitrite poisoning...
Place the sword off center, java moss on the branch would kick it up a notch, anubias and or java fern glued to the pots would help to transform from functional cave to inviting decor. Even as is, that is a very nice tank.
They are not immune, some species of cichlids need to be overstocked to avert aggression. In those type of tanks a large sump is used to increase the water volume, so a 150 gallon tank will often have almost the same amount for the sump...total water volume is doubled.
Bacterial diseases such as coulmaris / septicemia are brought on by overstocked tanks. Viral / iridovirus are common in dwarf gourami and overcrowding & the stress that it causes will weaken the fishes immune system, resulting in death. Overstocked tanks are not conducive to successful fish...
Well, a goldfish can get really big & when you say guppies they are also known as " the million fish" for a good reason, so being overstocked is quite possible. 5 guppies can become hundreds in a years time.
Everyone has given you excellent advise, I also am thinking it's a toxin. As a precaution Toss the food in the bin, replace with fresh, and use disposable food handlers gloves when you feed. Good luck.
Besides the wrong type of fish that are stocked, what i don't understand is how your ph could be so high when you are using distilled water? Distilled water is inert, and not suitable for fish or plants, it has no minerals. Good luck.
Is the total Gourami count 9 or ? Gourami don't like a lot of surface movement, so adding another filter could upset them. I would re home the SAE asap. When you have an overstocked tank everything may seem like it;s going fine, until it opens the door to some very ugly diseases. when the...
Did you use use Prime when you filled it on 8/27 so heavy metals & chloramine are now harmless? If you did, then add the TSS and the White clouds at the same time, sit back and enjoy! White Clouds are such great little fish, they never complain.