Fish are scratching

Hi all,
My aquarium: 200 Juwel Lido 200. Filtration: Oase BioMaster 600 (4kg of BiohomeUltimate), Sulfur Nitrate Reactor (around 1/2 kg of media) and a Eheim Surface Skimmer. Water change 30% weekly with 100% RO water remineralized with Seachem Equilibrium and KH Buffer, ratio 1 : 0.5.

Habitants : 10 sterbay corry
16 neon tetra
5 panda garra
6 amanno shrimp
20 - Galaxy Rasbora
2 - ottocinclus

My water parameters
PH: ~ 7.5
KH - 4 dKH
GH - 7-8 dGh
NH3: 0 ppm
NO2 : 0ppm
NO3 : <15ppm ( last evening was around 5 -10 ppm because on Sunday I did a water change)

Temperature: 25 degrees
Tank Age: aprox. 2 years

Why on earth my fish are scratching against the sand or the leaves ? I don't understand why this happens. Do you have any ideas ?

Hello and welcome to the forum!


Do they have small white spots on their body? If so, it’s most likely ich. Ich is a parasite that attaches itself to the fishes skin, sucking blood.

We can recommend treatment, if you do spot white spots.

I can't see any white spots. I might increase the temperature slightly to 26 to see if this will make it better.

MariusC said:

I can't see any white spots. I might increase the temperature slightly to 26 to see if this will make it better.

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When fish scratch, it usually means they have a skin irritation. (Aka ich)

I would keep an eye out for white spots, that’s my best guess.

I increased the temperature from 25 to 26 and I will wait to see if there are any significant changes. I noticed only the Panda garra and one Galaxy Rasobora ( that I forgot to add them into description ).

PS: I just saw that some of my corries are spawning again

Have you added anything new to the tank in the last few weeks?

Fish rub on objects if they have an external parasite or are exposed to poor water quality. External parasites are the most common.

Shine a torch on the fish after lights out and see if they have a yellow/ gold sheen over their body. This is velvet (Oodinium).

If they have small white dots about the size of a grain of salt over their body and fins, they have white spot (Ichthyophthirius).

If they have cream, white or grey patches on their body, they have Costia, Chilodonella or Trichodina.

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Velvet and white spot can be treated by raising the water temperature to 30C (86F) and keeping it there for 2 weeks. Increase aeration/ surface turbulence when doing this because warm water holds less oxygen than cool water.

If they have Costia, Chilodonella or Trichodina, add 2 heaped tablespoons or rock alt for every 20 litres of water. Keep the salt in there for 2-4 weeks.

To be honest, I added recently the 20 Galaxy Rasbora, but I had the same issue in the previous months. My water tests were good and the fish were scratching. I was doing water changes, but I want to understand the reason.

I am suspecting that my nitrate reactor is producing Hydrogensulphide , but there is no rotten eggs smell so I can't test.