Well, from the looks of it, and like FuzzyPeaches pointed out, there seems to be a water quality issue in your tank, and if that is something that goes on over time, then the fish will be effected. Some things fish do well with over a shorter period of time, like higher nitrates, other things do not sit well with fish at all, like the deadly ammonia and nitrites. Both latter ones shouldn't be present in a cycled tank, so something must have happened to your beneficial bacteria in your tank.
Do you have any gravel in your tank, or live plants? Dirty gravel, even if vacuumed on a weekly basis, can harbor all kinds of nasties, which effects your fish and your water readings. On the other hand, live plants use up nitrates as fertilizer, and tanks that are heavily planted can sometimes manage to come up with a zero nitrate reading.
I am not too familiar with the particular eclipse system you are using, but I remember it has a biowheel on it, right? Have you done anything to that biowheel lately, like rinsing/cleaning it under tap water? That wheel, as ugly as it might look over time, holds a HUGE amount of beneficial bacteria, and shouldn't be cleaned up too thoroughly. Especially not under tap water, since the chlorine in the tap water kills all beneficial bacteria. That goes for all filter media, btw - never clean it in tap water.
I also assume that the Eclipse 1 system is not strong enough for your roughly 20 gl tank, and your fish are big, which adds all together. Growing fish=more waste output, both via poop and every time they breathe, combined with a filter system that needs some help, plus the fact that water changes should be done more frequently, like more towards 25-50% weekly. Especially with a fish load like yours - you are pushing your limits there with 2 goldfish and the other 3. All of them are huge waste producers, and I will assume here that it all played a role in your fish's behavior.
What I would do if it were my fish is doing a big water change right now, even 70-80%. Remember, if you would do 50%, then you'll reduce your bad readings by only half, which still leaves you with a trace of ammonia, 100 of nitrates and 10 of nitrites. Any ammonia/nitrite readings above zero are toxic, and I would think that your fish are reacting to it.
Then I would go and find either a bigger eclipse system if you like that particular brand, or go for a more efficient filter alltogether. There are plenty of hang-on filters that are wonderful, as long as you remember that you need 10x the tank volume in g/h. So, with a 20 gl tank, you'd need a filter that AT LEAST pumps 200 gl/h, since your fish load is heavy.
Might be that with the inadequate housing your fish's immune system isn't up to par as it should be, and opportunistic parasites have gotten a hold of your fish. That would be where the fungus, bloody streaks (also a sign of inadequate water quality) and the tail issue are coming from. In a lot of cases, cure the water, and it'll cure your fish.
You can add some salt to the water to give the fish somewhat of a tonic - salt is also a great treatment for half a dozen kinds of parasites. Not any salt will do - most cooking salts have anticaking agents in them that is toxic to fish. Use aquarium salt, rock salt, or pickle salt. Any salt that contains salt, and nothing else. Right now, I have pickling salt ready in case something should pop up. Use 1 teaspoon per 1 gl of water, but beware that plecos are a bit sensitive to salt, so you might want to take them out first.
But remember, you can try to cure the effects, but as long as the water isn't taken care of, you'll have continued problems.