We're talking a 360 Liter tank. Depending on the length of the tank I wouldn't keep Melanotaenia boesemanni in it because in my opinion the tank doesn't offer the space they need. But if we go by bioload I would easily fit in 30-40 P. axelrodi and 15-20 Corydoras. Gregarious fish like these will always get enough food if fed correctly. Due to tetras and rainbowfish being quite aggressive feeders I would only worry about bottomdwellers. The food has to reach them, that is all you have to be aware of.Really? I worry about bioload and starvation even though it's 90 gallon tank. I thought more than 6 Corydoras might not get enough food even with sinking pellets. Same for Cardinals, the bigger the group some might miss out on food.
Up until there I'm with you.I have been at this nearly two years so I am new and still learning. I am very cautious about everything.
But there you might want to reconsider. Most of the fish we keep are small feeder fish in nature. They are many, they are the ones other species live off. They are hardier than you think if kept in the right conditions, which includes numbers. I do promote responsible, but also species appropriate fishkeeping. The numbers you are keeping are quite low and what I would mostly deem just about enough so the group sizes are not too small.They are living things I treat them like I would a dog.
But before I tell you to go and get more, how about you show us the tank, because a picture says more than a whole novel.
The only way to lower hardness is using soft source water. The stuff you add is just dechlorinator which detoxifies chlorine so fish won't die within minutes. 1 degree hardness is 17.8 mg/l (or ppm if you will), so you're actually at 19° hardness. One question is though, how much of it is GH and how much of it is KH.Even with Ultimate Conditioner or Prime it still only goes down to about 350 ppm so about 18 dh.
In your case if you wanted to lower this, you'd have to use an RO unit. Which I do not recommend, because of the overall volume of your tank, the fact you would have to replace the membrane in the unit at least once a year and that you have rainbow fish, which are actually hard water fish.
While softwater fish (like P. axelrodi, like Corydoras, like most dwarf cichlids) can live in it for quite some time, the conditions in hard water are excellent for opportunistic bacteria and these fish are very prone to bacterial infections because in their natural habitats the water is so soft, bacteria have a hard time surviving there. If a fish gets stressed too much the bacteria strike, cutting life-expectancy down by a bit. So if somebody is willing to keep these fish in such hard water, hygiene in terms of water changes (not cleaning, you can easily overclean a tank!) and water quality (nitrogen compounds and other waste products) is key. A well balanced combination of a high performance filter and lots of plants (including emersed plants rooting in the water) would be the minimum, a UVC sterilizer would be a nice-to-have additionally.