Yeips! Your water test results are not good. Ammonia and nitrite should always be at 0, and your nitrAte should be below 40 (unless it is higher than this in your tap water). Yes, you do need to do larger, more frequent water changes. Its hard to say exactly how much water to change, especially because your tank looks a little over stocked (I'll go into more details later). Right now, you are doing approx. 15% water change every other week. At a minimum, I would step this up to 25% weekly, because your tank is very over stocked (again, this is down farther).
Even though the nitrate is high, my big concern is the ammonia and nitrite. You need to begin doing daily water changes until both of these values are at zero. Now the question is WHY aren't these values zero? Your tank has been going long enough where it should be cycled (have the good bacteria to break the waste down) by now. Did you recently add more fish, clean out your spounge in tap water (or rinsing it really well in dechlorinated water can also do it), go a little too crazy with vacuming your gravel if you have an under gravel filter (UGF), never vacume your gravel if you have a UGF? If you have a hang on the back filter, does it have a spounge for the bacteria to live on, if not, did you just change your carbon filter?
Ok, onto your tank being over stocked. The general rule is 1 cm of fish for every 2 liters of water.
Here is your current stock --
Mollies = 2 x 10 cm= 20 cm
Yellow Jack Tetras = 4 x 3 cm = 12 cm (estimate, but tetras produce little waste)
Honey Dwarf Gourami = 2 x 6 cm = 12 cm
Platy = 6 x 5 cm = 30 cm
spotted catfish = 2 x 6 cm = 12 cm
Guppies = 4 x 6 cm = 24 cm
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Total = 110 cm
The max you should have is 35 cm in your tank. You really should look into rehoming some of your fish. I know alot of your fish may not be as big as the size listed, but this is their full adult size, which you need to plan for. Having your tank this over stocked will lead to having the poor water conditions you are experiencing right now, your fish will be more stressed, and also your fish will become sick alot easier. Your fish need room to swim around, and when they are packed in there, they don't get that.
I know earlier I told you to use antibiotics and salt. NEVER add salt to your main tank because of the catfish in there. They cannot tollerate salt, and are very sensitive to many medicines. good luck wiht your little guppy!
EDIT: After catching a stupid reading mistake, I don't think antibiotics are needed (mistake explained below)