Most Melanotaenia boesemani only reach 3-4 inches long. If you keep them in a 20 foot tank that is 4 feet high and live in Germany, they might hit 6-7 inches (but that was 20 yrs ago) but nowhere else in the world has seen them get that big.
Minimum size tank for them is 3 foot long and 4 foot + is better.
Red sores on rainbowfish is usually Fish TB (Mycobacteria as mentioned by CassCats).
There is no cure and you should assume all the fish in the tank have it, if it is Mycobacteria.
Feeding the fish lots of plant matter (at least 50% of the fish's diet should be plant based) can help stop the ulcers appearing on the body but the infection is usually inside the fish (in or on the organs) and it eventually kills them.
If the fish bloat up overnight, stop eating, do a string white poop, gasp at the surface or near a filter outlet, euthanise it immediately. These are the symptoms you see with rainbowfishes that are dying from TB and the fish are normally dead within 24 hours of showing these symptoms. Prior to that they will be swimming around normally, showing off, breeding, and eating. Then one day you wake up and find the fish with the above symptoms.
The links below have more information about Fish TB and how to deal with it, including information about people catching it.
So I have been reading things in Manual of Fish Health and this forum and have a bunch of questions regarding fish TB: 1. Why TB are common in fish from south/asian farm? 2. Can fish get TB even if you don’t buy new fish? 3. I always have cuts on my hand (I don’t know how?) How many percent can...
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So I’m not really understanding this, as it wasn’t around, when I had tanks 20 years ago ( or at least hadn’t been diagnosed ) Rumored to have come from Asian farming techniques… some fish species seem to be more susceptible to it than others ( my 1st experience, was with yellow axelrodi...
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so, with me giving up on rainbow fish, maybe Killi's ??? assume not in the same TB history tank... but these could certainly fulfill the color urge... um... maybe with a little more economical variety...
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a 4 footer is the minimum for boesemani and other large rainbows. Id only consider a 3 footer for ones such as praecox, sahulensis, maccullochi... always house as if it will reach max size instead of settling for "it doesn't normally grow that big". Your advice is insinuating fish grow to their environment which we already know is harmful to their health and wellbeing. 
