Is activated carbon bad for my surgeonfish in a marine aquarium?

Thanks for reaching out about your Oscar and including photos, but difficult to see the HLLE lesions due to too dark. The eye looks terrible and systemic antibiotics are recommended at the least. A veterinary visit is highly recommended to clean the wound and provide other possible treatments. The water quality must be kept pristine (pH, ammonia, nitrites, nitrates) to assist recovery. Antibiotics added to the water do not penetrate into the body, nor do fresh water fish drink, since osmotically water enters over mucous membranes (gills, mouth, skin) and antibiotics do not cross this barrier. HLLE causes can be multi-factorial including water quality, bacteria, parasites, carbon particles in activated charcoal (filters), low vitamin C, etc. Check and correct water quality, replace existing food if older than two months (vitamin C degrades quickly), see a veterinarian with fish experience. Good luck.

If you have an activated carbon filter insert, remove it or it will filter out the medication. Confirm adequate water quality and chemistry since both can impact health and healing. Consider adding a broad spectrum antibiotic to treat infection. You can purchase products such as Kanaplex or Maracyn in the aquarium section of the local pet or fish store. Follow label directions carefully. Plain aquarium salt (NOT table or Epsom salt) can be used at 1 teaspoon per gallon of water to reduce stress and aid in recovery.

It seems like there is an ulceration at the level of the gills. This may be related to a bacterial or fungal infection. Hole in the Head disease is possible as well. The exact cause of hole in the head fish disease has yet to be determined, but strong suspicion in terms of causes are parasite (Hexamita or Spironucleus infestation) or nutritional imbalances. Water quality must be checked and any abnormality corrected first. The treatment usually consists of taking a multi-faceted approach and includes includes the addition of the antibiotic metronidazole to the treatment tank housing the infected fish and making sure feeding is adequate. In cases where secondary bacterial infections are present, additional antibiotics such as Maracyn, Kanacyn, or Furan may be needed. When treating this or any disease, try to use a separate treatment tank and treat as soon as the first symptoms appear.

Hydrogen peroxide can be used but it has the potential to be somewhat harsh on many species of fish. Unless you need to use it for a specific reason such as to eliminate a particular organism that is susceptible to H2O2, I would shy away from this instead electing to use something designed specifically with aquatic animals in mind. Hope this makes sense.

Make sure the charcoal/carbon is removed from the filter/tank for the duration of treatment or it will remove/absorb the medication. Try using metronidazole and kanamycin. There is metronidazole in the Parasite Guard but it may not be an adequate dose. Monitor water quality and keep nitrates below 20ppm if possible. Use only aged/conditioned water that is temperature an pH matched to reduce or prevent stress. Raise the water temp to the upper limit for Oscars and add a multivitamin. You can soak pellets prior to feeding. If you can find it, use VitaChem (by Boyds). Make sure Tiger is eating a biologically appropriate diet.

Elevated ammonia levels can result in "burns" which may turn fins and scales black. Use a home test kit or submit a water sample for testing. Most pet or fish stores will do this for free or for a nominal fee. If ammonia levels are elevated, daily partial water changes, using aged or conditioned water, can decrease ammonia to safe levels (0.25 ppm or less). StressCoat or plain aquarium salt (NOT table salt) can aid in healing. Dissolve 1 teaspoon of salt per gallon of water into a cup of tank water and gently pour it into the tank. Confirm adequate filtration. The filter should be rated for at least twice the volume of the tank. You can change 20-30% of the tank water every couple of weeks. Use only aged or conditioned water that is temperature matched to prevent stress or shock.

Thanks for reaching out. Have you tested water quality for pH, ammonia, nitrite and nitrate. Correct problems. Don't see any signs of external lesion on the pictures, except fine erosion, and possible gravid fish. A veterinary visit is probably needed to trouble shoot for disease. Randomly adding chemicals to the water could have detrimental effects. Good luck.