Originally Posted by Matteoooooooo
I have 1 Swordtail, 2 Plecos, 3 snails, 1 North American Darter, 1 Blue Lyretail, 3 Silver Tipped Tetras, 2 Rasboras, 3 White Cloud Minnows
If I may suggest something, you should decide which way you want to go with fish . In a 40g tank you can have a truly wonderful display of fish and plants, with either basic harder water fish or soft acidic water fish. I do not recommend mixing the two when you have water as fine as yours seems to be.
First I'll explain what's occurring to lower your pH. As an aquarium becomes established or matures, certain biological processes settle down. Bacteria (and I am referring to more than just the familiar nitrification bacteria, there is also a host of bacteria residing in the substrate to break down waste into organics, some is aerobic (uses oxygen), some is anaerobic (very little or no oxygen). Plants are part of this biology and very useful for this reason. Anyway, as this occurs, the water acidifies and the pH lowers. The extent to which the pH will drop depends upon the KH (carbonate hardness) of the source water, the type and number of fish and plants, other water issues (minerals, organics, tannins) and how often you do partial water changes. We don't have the KH (hardness) of your tap water, but if the pH is 7 and your tank is lowering to 6.5 I would expect the KH in the tap water to be minimal, in other words, soft water. Weekly partial water changes of 30-50% will somewhat keep the water stable, as adding fresh water that is pH 7 to aquarium water that is say 6.4 will result in a slight increase in pH. But this must be consistent. This minimal fluctuation is not hurtful, it is completely natural in nature and occurs in all planted tanks with what we term the diurnal variation; but I won't go into all that.
So your source water (tap water) is probably soft. What this means is that you are ideally suited to soft acidic water fish. The tetras, Blue lyretail (a killifish, Fundulopanchax gardneri),
rasbora, and most likely the pleco are soft water fish, very much so. The white clouds are sort of "in between," they do well in either slightly acidic or slightly basic water [there's another issue here, in a moment]. The swordtail is definitely basic hard water and will not do well in soft water because it lacks minerals. It might be "OK" in your tank because we don't know the exact mineralization (hardness) of your water, so there is some leeway. The darter from NA could be either, I'd need to know the species and look it up.
The other issue with the white cloud (and the darter probably) is temperature; these are cool water fish, which is to say, room temperature is the highest it should go. Whereas the other fish are "tropical" and better at "normal" aquarium temps around 77-79F.
So, back to my original suggestion. Given the condition of your water, I would recommend you work towards a soft acidic water fish community. You have hundreds of fish options; all the South American and SE Asian forest fish (with very few exceptions), including the tetras, pencilfish, hatchetfish, catfish, loaches, rasbora, smaller barbs, gourami...the list is nearly endless. The aquarium will naturally, without intervention, provide ideal water, with the weekly partial water change.
If you have any questions, fire away.