Unknown particles on the water surface of my tank

[Blackwater guru]

So for a while now I have an issue with my 145 l tank that is powered by an undergravel filter.
The issue I have is that the water surface of my tank has been filled particles that look like this but if you saw it in real life it would look more brown.

I am suspecting that these are particles that come from when I vacuum the gravel during the water change .

Also recently one of my 14 cardinal tetras got sick lost colour and I found it dead while cleaning the tank today and it was already decomposing .

And no this is not a new tank because I have been running it for over a year with few incidents .

What is causing this and what should I do to fix this problem?

I also don't have a test kit to test for ammonia or nitrate so don't bother with asking me to test the water parameters.

 

[PorshaF]

It's hard to say what could have killed the cardinal without water parameters so i would go with a water change to dilute any issues in the water chemistry currently. What volume do you change weekly?

 

[Blackwater guru]

It's hard to say what could have killed the cardinal without water parameters so i would go with a water change to dilute any issues in the water chemistry currently. What volume do you change weekly?

I typically do a 25 % water change where the turned off water pump and pipes are exposed but not too much water is removed in a single water change and this is usually the amount I remove in a water change.

I currently have a 145 l tank at the moment but next time I visit my lfs I will buy a testing kit.

 

[Slaphppy7]

Do get the testing kit as soon as possible, IMO, it's critical for fishkeeping...get the API Freshwater Master Test kit.

UGFs are notorious for holding gunk and harboring high nitrAtes, simply because they can't easily be cleaned...they aren't used much anymore, because of this