I'd say that the tanks is about 30 gallons (UK). the tank is heavily stocked if my guestimations on dimensions are about right, Problems will arrise with the tiger barbs when they get a bit bigger and maybe the gourami, but all this doesn't help the cloudy situation.
Interpet doesn't seem to have a website I can find but I did find this
filter. could be that with two filters running the intake is too powerful and churning up debris in the substrate, but thats just a thought (Pete, Do you vac the gravel?)
Anyway I'm just up and will do a bit of checking later on and get back to you.
If the tanks up and running and has been for a year it should be cycled. If you've got any test kits it might be an idea to see what the ammonia, nitrite,nitrate etc levels are. Adding the plants will help the tank and the fish but I don't think will cure the problem.
is the cloudyness like a liquid's been added or can you see particles floating in the water?
Another thought, with all the fish in the tank could the population be almost all female and they seem to be spawning similtainiously? a shot in the dark but I'm running things through in my head.
Be back later.