My tiger barbs are looking a bit lethargic, water parameters are OK, what's wrong?

My tiger barbs are looking lethargic at the bottom of the fish tank. It's a 180 litre tank, with plants, and is well maintained. Water changes are 30% weekly. The parameters are 0ppm ammonia, 0ppm nitrite, PH 7, temp 25.5 degrees C, 30ppm Nitrates, and the water is well oxygenated due to a HOB filter, and an external filter. I have a co2 checker in the tank, which indicates the co2 is in the perfect range at 25. There is reasonable flow, but not extremely strong. Current fish stock in the 180 litre tank is, 3 yoyo loaches, 3 bristlenose catfish, 9 tiger barbs, 8 juvenile praecox rainbowfish, and 2 dwarf cichlids - male and female. The tank has an open space, and plants on the edge and back of the tank. It also has a clay pipe, My water changes are done with water conditioner (the correct amounts), and my T5 lights are on for 8 hours a day during daylight hours to mimic tropical conditions in the wild. The tiger barbs are 2 years old. I also use plant fertilizer in the tank (the proper stuff - easylife profito) which is a daily fertilizer, and I add only for the water volume in the tank. I also use seachem flourish, which is "liquid carbon" added daily. This has only happened in the last week or so since using plant fertilizer and I haven't heard of anyone who has any problems with these particular fertilizers after researching the web for 8 hours constantly. There is no excessive noise around the tank, and no banging or noisy distractions for the fish. My current stock is definitely not overstocked, as I've checked with professionals locally, and also on various advisor websites. My tiger barbs are mainly male, possibly I need to remove a couple of males and add a couple of females for more natural behaviour? Not sure on that one. I care about the welfare of my fish, and have worked hard to create a natural environment (and spent stupid amounts of money). What's going onnnnn?!