Dwarf gourami listless and not eating

[usnuzuluz]

I inherited a 30 gal established tank with 2 male dwarf gouramis (blue a little more possessive of his side of the tank than red) almost 5 months ago. A couple of weeks later, I introduced 1 albino corydora. After chasing the newcomer around for a while, things were fine. A few weeks later, I added 3 more albino corydoras. Again, everything O.K.
I vacuum, scrub and change 5 gallons of water every 2 weeks. On the 4th week, I also change the 2 filters that hang off the back of the tank (sorry, I don't know kinds -- it was all just thrown at me in one day).

Here's the recent problem: last week, 1 of the "newer" corydoras died. I hadn't seen all 4 together for a day or two and knew I hadn't sucked anyone up the hose when changing the water. Took the blob of fern out of the way the next day and found the dead one on the bottom. The following day I bought a replacement (same kind, same store). Yesterday I noticed the blue gourami is not interested in eating and stays at the top in a corner. Doesn't even both to chase "red" to his side of the tank. Will scurry away as usual when I put my hand on glass in front of him but returns soon after.
Everyone else in tank is acting normally.
I have noticed no spots or color change, no bloating, no tilting to a side, and no gulping.
In reading other forum messages, I am undecided as to whether there is even a problem at all. Old age perhaps? I'm guessing they're about 3 years?
I looked in the box of inherited stuff and pH bottle was empty so I guess I need to get more. I had taken the water to a pet store a couple of months ago and it was just slightly to alkaline side.
I know this is a little vague of the minute details you like, but the suggestions I've received from friends range from hexamite, use paragon, quarantine, too late -- put in freezer............
Help!

[Zagz]

Welcome to AA!

It may be old age or possibly by changing both filters at the same time, you are getting a mini cycle. Do you have any test kits to test for ammonia, nitrite and nitrates? A stable ph is better than trying to adjust ph, the fish will do better. First thing I'd recommend is to do a 50% pwc, it can't hurt and it might help.

[usnuzuluz]

Changing both filters was how the previous owner did it and instructed me to do it. It's been working fine so far. I forgot to mention food.
Everyone gets TetraColor tropical flakes once a day. Every third day or so just for a change, I give some spirulina flakes and a few freeze dried plankton.
Now that the light is off, I checked and blue is at top near filter. How I wish I was a mind reader......

[Zagz]

Is he gasping at all as if out of breath?

[usnuzuluz]

Does not appear to be in any distress.

[usnuzuluz]

Day 3
Still not eating; staying at top between glass and filter output.

[Zagz]

Have you tested for specific water parameters? Did you try doing a pwc? Have you noticed anyone bothering him?

[usnuzuluz]

No one is bothering him. Bought a "test it all" kit. PH was 7.6, ammonium 0, nitrite 0, nitrate 0.
Vacuumed and changed out 10 gal of water (30 gal tank). Retested with same results. Tested my filtered water -- pH is 7.4 but that's what they have been used to for 5 months.

[Zagz]

What kind of kit was it, test or liquid? In an established tank there should be some nitrates. Has he improved any since the water change?

[usnuzuluz]

Freshwater Master test kit -- yes, liquids and test tubes. VERY briefly after water change, he ventured to side and front of tank. But when making a turnaround, it was very rapid. This only occurred about 5 times and then he went to the bottom of the tank in the back underneath some moss.