1. One water change is good if the tank is fully stable/amtured water quality wise. However you should still test your tank for ammonia, nitrites and nitrates on a regular basis though which an accurate test kit/s in case any water quality problems do occur, you can deal with them imediately instead of let them go unnoticed.
Always use water conditioner/dechlorinator though on your tap water before it goes in the tank to remove the chlorine and any heavy metals etc in it from it. Never wash the filter sponge out in untreated tap water, only wash it out in old tank water to help prevent the filter bacteria living in the sponge dying off and causing water quality problems.
2. How large are each of your fish roughly and how long have you had them? Most of your fish are not very large growing ones either way, with the mollys being the largest growing (although probably won't exceed 3-4inches depending on your type of mollys).
Your fish are omnivores and insectivores, a varied diet consisting of both veg and high protein foods is best- there are a wide variety of foods you should be ble to get at your lfs. For example, algae wafers and catfish pelets are good for corys as these sinking foods are must more suited to them, while bloodworms, daphinia, krill and spinache are good for all fish (i would not personally advise tubifex as it has been known to come from sources contaminated with raw sewage, live tubifex can also pass certain parasites onto fish as well). Frozen or live foods are far better than dried ones as they are more nutritious and easier to digest for fish.
3. 25 degree's is an ideal temp for your fish.
4. For a 34gal tank it is not overstocked at all, but just to make sure it is 34gallons, what is the length width and height of the tank?
Although your tank is not overstocked, the stocking is still not completely desirable. Khuli loach should be in a group of at least 3, while mollys should be 2-3females per male or an all-male group of 5 or an all-female group of 3. Conga tetras are shoaling fish should be at least 5-6 ideally in your tank. Dwarf gourami's prefer a well/densely planted tank, so make sure you have a lot of planting with some clear open area's in the tank- tall and dense growing plants are the best
.