Colin has beaten me too it
But rather than retype all this lot (he posted while I was typing) it'll just re-inforce what Colin said.
First I would say stop using the pH down. It sounds as though you have hard water and also a high KH (carbonate hardness). This buffers the water and stops the pH changing. But not only that, fish that like lower pH also like water with few minerals in, and when you add the pH down you add more chemicals to the water, the opposite of what is needed.
If you want to lower the pH you also need to lower the KH. The way to do this is by mixing your tapwater with RO (reverse osmosis) water; that is water from which all the minerals have been removed. But you would need to use the exact same mixture at every water change of the fluctuating levels would harm the fish.
Re the fish you have, before getting any other species, you need to get more cherry barbs, kuhli loaches and cories of the same species as the ones you have. Both need to be in groups of at least 6. But the fish you have now put you at around 100% stocking. Even if you rehomed the cories (which probably grow a bit too big for a 70 litre tank) and got a couple more barbs and loaches, you would still be slightly overstocked.
It's not just about filtration capacity but space for the fish to behave naturally. Cories in particular need a lot of floor space which a 70 litre is unlikely to have.
Even if you has space, rams are not a good choice. They need soft acid water and temperatures higher than the rest of your fish.
As for shrimps, they add very little to the bioload so you could get some of them. I think that even the smaller species such as cherry shrimps should be OK with the fish you have. Or you could look at the larger amano or ghost shrimps. Whichever you decide, you need a group of them, and the tank should have some cover for the shrimps to hide in. They have to shed their skins to grow and until the new skin hardens they are very vulnerable to being eaten. They need to hide away somewhere during this time. It could be a cave or a dense plant.