Red streaks in the tail is usually some kind of infection. The most likely reason for this is water quality issues.
50l is a small tank for goldfish. It is just barely able to cope with 2 fantails. <A 20 or 30 gal
= 80-100l is much easier to manage.> The key to managing goldfish in small setups is water change. <Of course a bigger tank will be much better!> A better filter will help, but there is no getting away from pwc
's to remove toxin buildup.
Even though you have no measurable NH3
, other stuff (like dissolved organics) and nitrates do accumulate & weaken the fish to produce illness. If you have not done so, check the nitrate levels. Small goldfish tanks often have nitrates up in toxic levels (like 100 or 200). Also check the pH compared to tap ... again, a lowered pH is another indicator of toxin accumulation.
Even if all tested out fine, I would increase the water change amount. For a small tank like yours, i would do 50% pwc
weekly at a minimum, if not twice a week. <Fanatics will do daily pwc
's in small setups.> Clean water is your first defense against disease.
Although it is tempting to just treat the symptoms, it is typically useless unless there is pristine water conditions. As for what the infection may be, the most likely is an internal bacterial infection. <Usually an opportunistic bacteria that attacks weakened fish ... 5-6 year old is old in terms of fancy goldfish (typical life span is 7-10 years) so immunity is likely less than the new fish.> After fixing the water, I would be tempted to treat either with an oral antibiotic (ie antibiotic in food). External treatment tend not to work well for this as the drug won't get into the fish (goldfish being thick bodied). The only role for external treatment is if you suspect some external parasite that created a wound to allow for bacteria entry. <For this I usu. just use salt, but salt will kill your plants at doses needed - 0.1% to 0.3%. Of course, you can remove the plants to a holding tank during treatment.>
If you do decide on antibiotics, try to find food that contain kanamycin. That is the current best treatment for goldfish.