'Fish Swimming At The Top Of The Tank

I have a 25 Liter long tank. It has lots of plants (healthy), a cory, and 4 mollies. This morning, all of my fish except the cory were swimming at the top of the tank but not in a healthy manner. Their color was off and they looked like they were gasping for air. 

I have had the tank for a month and a half. Last night I did a water change (about 30%). I used Prime to condition the water, it was the same temp as the tank and I vacuumed up some of the waste in the gravel. I also added Stability after the change as I had heard water changes in a nano tank like this can upset the nitrite/nitrate balance. I use Seachem Flourish and Flourish Excel per the local fish stores recommendations for the plants in the tank.

So I made another 30% change this morning and the fish seem a bit better. Any idea what may be happening?

The tank has been cycled. It's about 45 days old. I can't see colors so the manual test kits don't work for me. I know there are electronic ones but at $40-$50 US a piece it's a bit beyond me right now. I have been taking water samples to the fish store but they charge me $3 per test so its $12 for all the tests. They are closed today or I would have forked over the money to find out. Its just odd that all of a sudden they are swimming at the top. Is there an issue with oxygen at night? I understand that at night plants take up oxygen but in the day that take up CO2.

Gasping at the top is lack of oxygen in the water or ammonia and/or nitrite in the water. If you can't judge the colours yourself, take a sample to a fish shop and ask them to test for those two - but get them to write down the actual numbers, not something like 'a bit high' or 'OK'.

It took me 42 days last year to cycle a tank, this is fairly typical. Can I ask how exactly you cycled it since you can't use test kits? The only way to know a tank is cycled is when both ammonia and nitrite stay at zero.

And the bad news is that mollies are totally unsuitable for 25 litres, I'm afraid. They need at least 100 litres as the females can grow to around 6 inches/15cm. The cory also needs to be in a shoal of at least 6, and a 25 litre tank can only accomodate one of the dwarf species.
Sorry to be the bearer of bad news 

For a 25 litre tank, you need to be looking at a single betta, nothing else; or one shoal of fish that grow no bigger than 2cm at full adult size.

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.