It's a long one but I'll be as detailed as possible in the hopes that someone will have some insight. I've been struggling with this for months. I'm at my wit's end and fear I'm losing the battle.
First, the story.
Two goldfish, Big and Small, one small snail. Have had them about five years. They started out in a 2.5 gal Fluval then about six months ago, moved up to a 5 gal PetSmart that was really bad. So moved up to a 9 gal Fluval Flex. I know this is a bit small but we're now maxed out on space. One of the things that prompted the attempt to get to a bigger tank was that Big recently started bullying Small after being fine with each other for years. I'd put Big in a small bowl for a couple hours until it had calmed down then back in the main tank and all would be well again for a day or a week or two weeks or some seemingly random period of time. During one of those transfers, Big must have struggled in the net too much and injured a back tail fin - that's when the problem started.
That injured fin on Big developed red streaks that ultimately spread to Small. I initially tried API Melafix for 2 - 3 weeks and that didn't help. Then I tried API Fin and Body Cure (doxycycline). The antibiotic helped but the problem just came back. I tried it a second time and it got better then came back. I don't know about dosing fish with this kind of thing, if I should just keep up the med until the streaks are completely gone no matter how long it takes? After the second course of the antibiotic, I started daily Melafix and Pimafix. That's now been about four weeks ago and the problem is still slowly coming back.
Some observations that I think may be related:
Big bullying Small gets worse right after a water change / tank clean. The bullying happens only in the mornings. I've tried changing different amounts from 25% to 75% of the water. Seems like more water change helps the red streaks but makes Big more of a bully & vice versa but this is a tentative observation. When changing the water, I also clean the glass with a brush and rinse the filters. Filter media gets changed like this: charcoal once a month, Fluval Biomax every six months, API Nitrazorb every six weeks. When changing the water, I treat it with API Quick Start, API Stress Coat, and Tetra AquaSafe. I also add about a half teaspoon of API freshwater aquarium salt.
Maybe because of all the activity, they recently get anxious when someone comes by the tank. They dart around, eventually hiding behind the plant and/or sticking close to each other.
Here's the Help Request Form info as tested last night with the API Master Test Kit:
Ammonia Level(Tank): 0
Nitrite Level(Tank): 0
Nitrate level(Tank): 20
Ammonia Level(Tap): Not tested
Nitrite Level(Tap): Not tested
Nitrate level(Tap): Not tested
Ph Level, Tank (If possible, KH, GH and chloramines): 7.5
Ph Level, Tap (If possible, KH, GH and chloramines): Not tested
Other Required Info:
Brand of test-kit used and whether strips or drops? API Master Test Kit - drops
Water temperature? 78F
Tank size (how many gals.) and how long has it been running? 9 Gal, about six weeks. I added an air stone that didn't come with the tank. All tubing is clean. I noticed the OEM tubing that comes with things is of poor quality.
What is the name and "size of the filter"(s)? Built-into the Fluval Flex.
How often do you change the water and how much? Weekly, have tried 25% , 50% 75%
How many days ago was the last water change and how much did you change? Two days ago, about 30%
How many fish in the tank and their size? Two goldfish: Small = about 2.5", Big = about 4.5"
What kind of water additives or conditioners? API Quick Start, API Stress Coat, Tetra AquaSafe, API Fresh Water Aquarium Salt
What do you feed your fish and how often? TetraFin Goldfish Flakes & Aqueon Goldfish Granules, morning & evening. They eat all in a couple minutes.
Any new fish added to the tank? No
Any medications added to the tank? Currently API Melafix & Pimafix daily.
List entire medication/treatment history for fish and tank. Please include salt, Prazi, PP, etc and the approximate time and duration of treatment. In the story above as best as I can recall.
Any unusual findings on the fish such as "grains of salt," bloody streaks, frayed fins or fungus? Just the red streaks on their tails.
Any unusual behavior like staying at the bottom, not eating, etc.? I came to use the Nitrazorb because of Nitrate build up that would make them languish around the bottom. Since using that, they're back to their normal happy selves and Nitrates are under control. The red streaks don't seem to be bothering them. Their appetite is same as it's always been. They're excited for feeding time. They do recently seem scared of people but that goes up & down. Big bullies Small sometimes a morning or two after a water change - no idea why.