Alright, let me first apologize as I only read a little of what was posted and may have overlooked a few things. I noticed you said you treated your water once with some anti-bacterial meds.
All anti-bacterial meds target bacteria and can disrupt your "good bacteria" - specifically, the ones involved in your nitrogen cycle. In short, one way to deal with a hampered system after using medications, you can get yourself some more bacteria in a bottle - check out Amazon.com: API Quick Start Water Conditioner for Aquariums, 16-Ounce: Pet Supplies
. Also, before you resort to medications next time, use aquarium salt. It's a general treatment and may solve your fish health problems without the costly expense of medications and the adverse effects on your biological filtration. Also, water changes. If that doesn't solve your issue, then go onto the strong stuff like anti-bacterial meds. I also noticed you mentioned something about your pH dropping suddenly.
What I didn't see was where is the water source you're using when doing water changes? I'm assuming tap water and not bottled water or something else? A sudden drop in pH could be due to a lack of minerals, I forget what it's called, general hardness? gH/kH one of the two. Anyway, you can add something like limestone or crushed coral to raise the pH in your tank and keep it more stable at a higher number. Gravel, like those chunky rocks, are also for this purpose.
You could chemically treat your water source (I'm assuming you're getting it from tap water) with "pH Up" or whatever product in a bucket before putting it in the tank. You can also just stick on a hang-on-filter and run the water over some limestone for the same effect. For further water clarity
Do you use activated carbon in your filters? It can remove the tiniest of particles and help clarify water generally. You can also add finer sponges in your filters (polisher
). Lastly, you can always add more filters (but I'm sure you have plenty already). Finally
Have you tested your water source against your tank source?