Here is the deal with fish. Not just puffers but all fish in general: The early growth period, up to the point where they hit sexual maturity, is the most important in their lives. If they do not get a good start in terms of nutrition, environment, etc., then they just stunt. And that is permanent. It would be like not feeding a child properly during the growing years. We could not reasonably expect such a child to grow into a healthy adult and to live a normal lifespan. The same tings happens to fish.
Now, obviously, none of us knows the history of your little guy prior to when you got him but we do know that parasites are part of the equation, which certainly is not helping matters. I will go out on a limb and say that your guy probably did not get optimal care prior to when you had him. I have seen lots of Fahaka and (less) Mbu in local pet stores and have yet to see one which I considered to look "really well cared for". In other words, the longer they stay in that dealer's tanks, the more downhill they will slide. That's a possible scenario for you. Your puffers were lingering around for a long time, somewhat wasting away, prior to when you got them so of course they never grow well. Again, this is all speculation in regards to your fish but it is very plausible scenario.
It is entirely possible that you are/will do everything right and still just have this runt puffer. As they say, it is what it is. Just my thoughts.
Now, obviously, none of us knows the history of your little guy prior to when you got him but we do know that parasites are part of the equation, which certainly is not helping matters. I will go out on a limb and say that your guy probably did not get optimal care prior to when you had him. I have seen lots of Fahaka and (less) Mbu in local pet stores and have yet to see one which I considered to look "really well cared for". In other words, the longer they stay in that dealer's tanks, the more downhill they will slide. That's a possible scenario for you. Your puffers were lingering around for a long time, somewhat wasting away, prior to when you got them so of course they never grow well. Again, this is all speculation in regards to your fish but it is very plausible scenario.
It is entirely possible that you are/will do everything right and still just have this runt puffer. As they say, it is what it is. Just my thoughts.