OP says scrape, looks like scrape to me, but I also see what appears to be a hole/pit/ which could mean the beginning of HITH. Good catch.
IMO you have a combination of things going on here, Dave has covered the bulk of it. Water too warm, water quality poor (40 ppm nitrate is typically far from ideal), the fish are too crowded, and their diet is poor. The more the fish grow the tighter the fit, and the poorer water quality will become. I don't care if you do 50% water change every day, this is too much fish for a 150.
Invest in a bigger tank, or thin out your fish. That may sound harsh, but even with increased water changes the stress of living nose to nose over time will wear on a fish. You have a family of 6, living in a 2 bedroom condo.
Keep water in check @ 20 ppm nitrates or lower, I shoot for 5 ppm in my tanks, not because I am concerned specifically about nitrates, but they are a good measuring stick for other things, such as the overall bacteria load in the system. Keep on top of filter media cleaning, this too can lead to increased nitrates etc, all that waste builds up over time and needs to be kept in check, as much as the water.
Lastly, improve the diet. Tilapia alone is probably the worst part of this scenario, besides amino acids (protein) and fatty acids (fat), it is seriously lacking in nutrients. Vitachem that Dave mentions is good, but for proper absorption by a fish it really needs to be used as a presoak for the food, it has very little value when added directly to the tank water, and IMO doing so is a waste of $$$$. I have used this product for decades, with species that could not be trained to eat commercial food, but have not used it for several yrs so I have no idea what the current formula covers, or if it is the same as when I used it in the past. There are other formulas on the market as well, whatever you buy choose one that covers a broad base of vitamins/minerals.Any decent commercial brand of pellet will contain the proper vitamin/mineral premix, but if you have to stuff the fish with pellets I would go with NLS, or with Northfin as a second choice. Both are premium foods, the former has the best vitamin/mineral profile on the market.
Sometimes one has no choice but to feed fresh/frozen, which is fine as well, but if that is the case you need to follow the lead of public aquariums and zoos, and supplement the diet with vitamins/minerals. As an example, teleost fish cannot synthesize vitamin C, they need to receive this nutrient via their diet on a regular basis. Failure to do so can lead to all sorts of health issues, and this is also the key nutrient that a fish requires to fight stress.
Your fish are stressed, which is why they are showing "off" coloration.
So the #1 nutrient that they require, is completely lacking in your diet.
Good nutrition in aquaculture systems is essential to economically produce a healthy and high quality product, writes Mr Prakash Chandra Behera, Technical Manager (Aqua Division), PVS Group, India.
thefishsite.com
HTH