Arkangel, I know that in another thread you said that you wanted to do a ram tank in your 75g, but I think you should do something else with it and I'm really surprised that nobody has pointed it out thusfar.
Looking at your 180g tank you have many discus in there of varying sizes. I see that at least one of them is showing signs of being stunted. What I recommend to you is that you make your 75g a barebottom tank, get the temp up to 86 degrees, and move all of the fish that are smaller than 4.5" TL over to that tank to grow them out. They aren't going to be the beautiful adults like you have in there now as the adults will outcompete them for food and juvies need TONS of freshwater to grow. Hormones are secreted by the fish (especially the more mature fish) that prevent smaller fish from growing when the levels get to a certain point. This is why juvies need large and frequent water changes. I suggest that you move them over and keep them there for 3-4 months until they are 4.5" or so (not including the tail length). Feed them high quality and nutritious foods and you will be amazed how much they grow from this and the large water changes you should be doing. I've seen a hobbyist document with pictures the growth rates of fish that were stunted (far worse shape than yours) and they turned out to be some very nice looking fish. He did this with large water changes and at least half a dozen feedings per day. I'm not sure if the number is right but I have in my mind that he fed up to 11 times per day. This is why a good quality flake like Ocen Nutrition Prime Reef will come in handy for your situation. It will help your juvies grow into the beautiful adults that are already in your tank.
Looking at your 180g tank you have many discus in there of varying sizes. I see that at least one of them is showing signs of being stunted. What I recommend to you is that you make your 75g a barebottom tank, get the temp up to 86 degrees, and move all of the fish that are smaller than 4.5" TL over to that tank to grow them out. They aren't going to be the beautiful adults like you have in there now as the adults will outcompete them for food and juvies need TONS of freshwater to grow. Hormones are secreted by the fish (especially the more mature fish) that prevent smaller fish from growing when the levels get to a certain point. This is why juvies need large and frequent water changes. I suggest that you move them over and keep them there for 3-4 months until they are 4.5" or so (not including the tail length). Feed them high quality and nutritious foods and you will be amazed how much they grow from this and the large water changes you should be doing. I've seen a hobbyist document with pictures the growth rates of fish that were stunted (far worse shape than yours) and they turned out to be some very nice looking fish. He did this with large water changes and at least half a dozen feedings per day. I'm not sure if the number is right but I have in my mind that he fed up to 11 times per day. This is why a good quality flake like Ocen Nutrition Prime Reef will come in handy for your situation. It will help your juvies grow into the beautiful adults that are already in your tank.