discus, especiallyb abies at 28c temperature will eat voraciuosly no matter the space of tank.
its a matter of growth, stunting and ruining them.
if you buy a high prized discus.
I have purchased rare discus before at 250 each! medium youngh adults....
you spend that sort of money you wont wanna stunt them.
you may buy smaller ones, that are cheaper.
when you buy smaller discus, you already hav a large, very lage chance they will never grow to full potential.
Only a small proportion of discus from a batch of fry will actually look great!
alot will die, probbly almost if not mroe than half.
thats why adult discus, taht look spectacular you pay more for.
your case, if you add more to your 150L, you will regret it.
you will stunt them, though yoru aily water changes are great, puttin 40% cold water into a warm tank will reduce temps that will impact your discus too (unless you are pre heating the water or mixing hot and cold from tank) but what ah assle.
better to have more volume and save allt hat ime.
IF you havea space constraint. then i suggest you reasses the number of fish - in all honestly, a 4 foot tall tank will be hard to manage, you will need to ensure that that tank is on a extremely low stand.