Mysterious deaths of fish and plants looking terrible

[YourPalCM]

My Dad has tried to join me in my love of aquariums and has had a 40g freshwater tank set up for 2 years. He had about (3) Gouramis, about (7) very small Tetras, and a few bottom feeders. Overstocked a bit, but I gotta say they were thriving for about 18 months.

It's also a beautifully planted aquarium.

A few months ago his plants started looking terrible, and he lost (3) fish. The plants eventually sprung back to life.

All of a sudden his plants seemed to start to look awful again, and he's had another rash of fish deaths. He lost about (6) fish over the last week, and there are a couple others that don't seem to be eating.

We tested for all of the basics. pH he keeps around 6.7 using a CO2 tank, Ammonia is 0, Nitrites are 0, Nitrates are in an acceptable range. Temperature is around 76-77.

Should he be worried that something is suddenly in his municipal tap water that he hasn't tested for?

He changes 50% of the water each week because it's right next to a sink and easy to do. He's going to scale back to a normal 20% and start using a stresscoat.

He doesn't clean the tank glass with anything but a wet cloth, so he's not getting any cleaner in the tank.

Thoughts?

 

[Jchillin]

Nitrates are in an acceptable range. Temperature is around 76-77.

It would be beneficial to know the exact level of nitrate. The temp is a bit low, I keep mine at 81-84d at all times.

It would also help to know which fish have succumbed and what was the period of time between these deaths. I wouldn't look at something from outside just yet, sometimes fish expire due to age or stress. The wet cloth concerns me a bit, only because there are dyes in the material nothing more. Paper towels are a bit better for glass cleaning.

 

[Wizzard~Of~Ozz]

but I gotta say they were thriving for about 18 months.

If the light bulbs are T8 or T12 they need to be replaced every year. The output from them stops benefiting the plants and the plants will die off (and when plants stop consuming nutrients, they build up, and nitrate can become toxic (esp. in a slightly overstocked tank that would have been fine because of the plants), they will even start producing nitrates in the form of decaying plant matter)