I agree with what essjay, munroco and seangee have posted, and the data from PheonixKing.
The Tiger Barbs need a group of at least 12 (you have the space for a larger group, up to you) and they should be added together, so with five now you need to acquire 7+ ASAP. This really does make a difference to this species. Just keep in mind though, that with this species, which has a tendency to nip fins on slower less active more sedate fish, you are limiting your tankmate options. Cichlids and gouramis are out. That still leaves other cyprinids like barbs and danios, and some of the loaches, and active species of tetra.
To the loaches...this is a highly social fish (all loach species are) and a group of at least five is minimum, preferably one or two more. You have space for many of the botine species, there are several that are relatively peaceful. The clown loach though is not going to work here, the tank is way too small. I know the fish in stores seem "small" but they are juvenile, not mature fish, and as these begin growing they will reach 8 inches (20 cm) within just a few months (assuming they are in a group and healthy) and can then increase to 12 inches (30 cm). They have a strong social order, are quite rambunctious, and certainly not suited to many home aquariums. Botia kubotai, B. striata, and a couple others are suitable here, or one of the "dwarf" species. Loaches need lots of chunks of wood or similar structures, as each loach needs to select his or her "home."
To your question of the pH lowering...pH is part of the GH and KH triangle. The GH and KH will usually remain close to the source water, but pH can lower depending upon the GH and KH, and other factors like CO2. The GH is soft to very soft, and the KH is also low; nothing wrong with this, it is suited to most soft water fish species. But please do not acquire any livebearers (mollies, swordtails, platies, guppies, Endlers) because they must have moderately hard or harder water to function properly. The pH, given the GH and KH, will tend to lower as organics build in the substrate from the fish being fed, no problem there either; most soft water fish like acidic water.
Water changes...they must be more substantial. Once each week, change at least half the tank volume (at one go, not spread out). As the fish load increases, you can increase the volume of the regular weekly W/C to 60-70% which is where it should be in any fish tank. The fish will be healthier, guaranteed.