I keep seeing you write "until they outgrow this, which will be a long time". Well no, it doesn't work like that, fish need to be in a tank suitable for their adult size from the start as an environment that is too small for them can lead to stunting. I think this is where you got the idea that they grow to the size of the tank. It's incredibly unhealthy for them, drastically shortens their lifespan and affects their health longerm. In my experience fish grow to their full size very rapidly given enough space. I think you need to approach stocking a bit differently, youre sticking to things that are myths and misconceptions, at 18 you can't have been doing this as long as many of the people on here which are giving you sensible advice.