Diagnosing disease/fish problems is not always simple, as so many things can affect fish and produce similar symptoms. I seem to write it in every post lately, but the relationship of fish to their aquatic environment is very much more significant than that of land animals to air; water entering the fish is much different from air entering the lungs, and everything in the water gets inside the fish, dispersed through the bloodstream and into organs--and most all of these substances probably has some effect which will be detrimental and certainly never beneficial (except treatments for specific disease obviously).
From the information you've provided, I will only point out things I see as problems, without meaning to suggest any one is the direct cause of the deaths/behaviour. But cumulatively they can be so.
I would first recommend never adding substances to the tank water when you have done a partial water change, other than conditioner of course. These substances sometimes interact in ways we cannot expect. Conditioner is necessary if chlorine and/or chloramine is present in the source water as it is for most of us. Which conditioner you use should always be based on what you need to deal with the water, nothing more. For example, Prime detoxifies ammonia, nitrite and nitrate; unless any one or more of these is present in the source water, this is not a conditioner to be using. Seachem themselves cannot explain how Prime deals with nitrate. It binds nitrite somehow, but in any case this binding is only temporary, for 24-36 hours, whether ammonia, nitrite or nitrate. There is no point in clogging up the fish's bloodstream with needless chemicals or substances that may start doing something else. I know Prime does react with other substances, this has been documented in other threads, but at the moment I cannot recall the exact substances so let's just recognize the possibility and avoid the situation.
EasyLife Profito is a "comprehensive" type of plant additive, and should be safe but be very careful not to overdose, and it is generally safer to under-dose and work up to the recommended dose at first. There is no point in adding more than what is actually required of any of these plant products, same reason as above--the fish. If this were accidentally overdosed, it might well cause distress for fish. If used, I would dose the tank on the day following the water change, and less than full strength, even using two doses a few days apart. I've found this works better with other fertilizers like Flourish Comprehensive. For one thing, the heavy metal detoxification in most conditioners will negate some of the minerals in the plant supplements so they are best used apart.
EasyLife liquid carbon I will assume is their EasyCarbo. They don't say the ingredients, which bothers me. The fact that it kills algae suggests some toxic chemical, perhaps glutaraldehyde (this is the ingredient in Seachem's Excel and API's CO2 Booster). This stuff may be dangerous, and doesn't belong inside fish.
You mentioned a blue ram dying. This species needs warmth, minimum 80F/27C or it will be weakened and thus susceptible to other issues. They never live their normal lifespan at cooler temperatures. However, this warmth is not appreciated by some of the other species mentioned, so for their benefit I would not replace the ram. Now, 1 or 2 degrees may not seem like much, but it is with fish; they are ectothermic, and the water temperature drives their metabolism directly.
There are livebearers mentioned, mollies and platies. These need moderately hard or harder water, the mollies in particular. The other species are softer water, though some have a certain level of tolerance. But rummynose tetra will never be "healthy" in water hard enough for mollies to be "healthy," so here there is a real problem. Without knowing the GH, Kh and pH I can't offer much more. But if these values are not suited to the species, it weakens them long-term, and again this is just one more factor into a fish not coping with something else.
The angelfish, as two were present...was this a bonded pair, or two females? If two males they would not get along at all, and a male/female might for a time or bond, whichever. My thinking here is that something like this again weakens fish so it is a factor.
The cories should be increased, they are highly social and larger groups are better and results in less stress which again is a factor.
I hope some of this helps your understanding, feel free to question anything.
Byron.
