I would stop using any buffer until you know the cause.
A drop like you had is usually caused by having water that is very low in KH, ie carbonate hardness. Carbonate buffers the water against changes in pH and with water that has low KH, there is very little to stop the pH changing.
Have a look at your water company's website. They should give your hardness on there somewhere. That will be general hardness, GH, rather than KH but if your GH is low the chances your carbonate hardness will also be low.
I'm surprised at the reading of 5 for nitrate with all the fish you had in there. I would have expected it to be much higher, though if you had a lot of plants they would use both ammonium made by the fish and nitrate made by the filter bacteria from any ammonia the plants didn't use. But for this to happen you do need a lot of plants.
As for the plants using the oxygen, they do use it at night but they make it during the day. Well, they use it during the day as well but they make more than they use so the overall effect is more oxygen in the water.
Air stones only help by churning up the water so it can absorb oxygen faster. A filter positioned so the outflow ripples the surface will do this better.
I know you said you had trouble reading the colours of the test kits, but is there anyone who could read them for you? You really need to check your water parameters yourself rather than relying on a shop to test it for you. If there is someone who would help, get yourself a liquid reagent kit, the kind with test tubes, and a bottle of ammonia and add enough ammonia to give 3ppm. Then test the following day. If you have any reading for ammonia and nitrite above zero, what killed the fish was an overstocked, uncycled tank. For a 25 litre tank, if the ammonia is 9.5% you need to add 0.75ml. A syringe would make it easier to measure.
And you need to think about what fish are suitable for your tank. There are no fish that grow to above 2.5cm that are suitable except for one siamese fighting fish. Most 'not so delicate' fish tend to grow larger than 2.5cm.