I had that on my tank before but it was under the light just above the water. I did a NitrAte test and it was 80ppm for me so I did a water change. I think I was over feeding and the waste built up and caused the spike.
Could be because your over feeding and you think its gone because the fish search for food but I am sure it is not all gone and then rots in there.
I usually do a water change once a week or every 2 weeks now, I used to wait a month but that was to long for me. I do a gravel cleaning once a month.
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38 Gal Tank with Emperor 280 Bio-Wheel
150 watt Visi-Therm Deluxe Heater
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3 Red Serpae Tetra
3 Blackskirt Tetra
3 Lamp-Eye Tetra
3 Red Eye Tetra
2 Rummy Nose Tetra
4 Zebra Danio
3 Lemon Tetra
2 Otocinclus Affinis suckermouth (Large)
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