It's hard to tell from your photo exactly what those 'white marks' you're describing actually are. Plus, the Golden Severum is very light in color as well, so that doesn't help much.
But if you're seeing white marks where there weren't any before on the fish, that could be an indication your fish has a fungus infection. It could also mean that this Severum in the early stages of Head in Hole/Head and Lateral Line Erosion (HiH/HLLE) disease. I can't tell exactly what it is from the photo.
Regardless, the causes of both conditions are almost always poor water quality in the aquarium. Do you have any way to test for Nitrates in your aquarium water? Nitrate buildup occurs when there is not enough water changes happening to dilute those toxins out of the aquarium water. Your aquarium appears to be heavily stocked with fish. More fish, (and bigger fish) will cause Nitrates to buildup faster in the aquarium.
But, without a Nitrate test kit, it will be difficult to determine how high the Nitrates have built up in your aquarium. A maximum level of Nitrates is 20-30 PPM (better if kept lower than that). With levels higher than that, the fish can get stressed and all sorts of bad problems like fungus infections or HiH/HLLE can occur.
To treat this,
- First clean the aquarium and water. Clean and wipe down the interior walls of the aquarium. Remove all furnishings and scrub them out. Then you will need to start doing water changes, and if you have gravel it will need to be vacuumed carefully. If it has been awhile since a water change was done on the aquarium... you will need to start slowly to avoid shocking your fish. To begin, I would recommend no more than 25 to 30 percent water changes done daily for three days. After three days, move the daily water change percentage up to 50 percent, and conduct those 50 percent water changes for 2 days. After that, your fish will be ready for high-volume water changes and you will then conduct 75 to 80 percent water changes daily for three days.
- Initially, add 1/3'rd of a cup of non-iodized salt for every 25 gallons of water in the aquarium. After that initial salt dose, add salt to replace the amount drained out in water changes. After treatment is over, simply quit adding salt and it will be diluted out of the aquarium water through water changes.
If the Severum doesn't improve after cleaning the tank, doing daily water changes and adding salt to the water? You will need to isolate the sick fish and treat it with a medication. Use 'IchX', or 'Maracyn' to treat it, and follow the directions on the container.
I think its hole in the head.I dont have test kit and i think my tank is overstocked.First i will remove some fish tomorrow and then do regular water change and add salt for somedays.normally i do 50% weekly.
I am new to this hobby.how much medium size fishes should be fine in a 40 gallon tank.
Any more tips....
It may be best to think of your 40 gallon tank as a 'grow out' tank for young, juvenile Cichlids. A thing that is pretty good now for initially keeping these little fish, but not for long term. So, what will work long term in a 40 gallon aquarium?
A pair of Severum, or a pair of Convicts.
- or -
Three unpaired Cichlids that are no bigger than the size of an adult Severum.
And, that's it!
When starting out, a 40 gallon tank seems pretty big. And it is a lot of water! And initially, when first getting these baby-sized Cichlids - the tank visually looks plenty big enough. But unfortunately, that isn't how this works. To achieve all of those fantastic colors, shapes and other things that made those New World Cichlids so desirable in the first place, some adult size must be attained. And no, keeping them in a small tank does not keep these Cichlids small.
They just keep growing until the water quality gets so bad the fish start getting sick. Or alternatively, the aggression mounts up so high in the small tank that the more pugnacious species wind up actively killing everyone else kept with them.
To succeed in this with the Cichlids you have chosen to keep, you will need less fish and/or larger aquarium(s).