Rosejk,
Ok, this is going to be another 'in response to your previous two messages' post. I will try to cover everything.
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Ahh, Obsidian, you are my fish-guru-hero guy!!!!!(i think guy, i don't know many woman who would give themselves such a name, sorry if I am wrong tho!)
I am actually a girl

... I go by obsidian or obsidiana on a bunch of sites, its just an uncommon name that usually no one has used that I can get away with... but you're right, it does sound like a guy.
I wasn't able to find to much about Nitrofura-G on the web, but I did find the following: Nitrofura-G- This is good for hemorrhaging and red streaks, fin and tail deterioration, open sores, as well as general bacterial infections and gill diseases.
The maracyn was wickedly over priced! I can usually get it for less than $10 US.
From the description, it seems as if its ok, I would look for improvement in your fishies condition with the meds. A good way to 'track' progress would be to take pictures if you have a digital camera. Its hard to see if they have gotten better day to day, especially if you are checking on them frequently.... gradual changes just don't show up.
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so I emptied about 10-15 gallons of the water in the 75 gallon so that I didn't have to buy a 3rd pack.
I would defintely keep an eye on your water parameters, since with less water there will be more problems keeping them under control. If there are spikes, do a water change and just put back in the appropriate amount back in of meds.
As for the med $, oh yes, it is shocking how much a little fish can drain your pocketbook when its sick *sigh*
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Anyways, the news is, I started treating last night, there's 14 fish in the 75 gallon and they all have the red streaks in their tails, except for the pleco and the 2 loaches
The loaches and pleco will probably not show too many symptoms, I have never had them affected even when I had fish die from an ammonia spike.
For bob, the heater is fine... actually goldfish are a 'coldwater' fish, and they can handle low temps, even slightly above freezing. And good about the airstone. Good to hear bob is much better. The meds treating for 3 times every other day is common. My Kanacyn is the same way, I *think* that it gives the fish a chance to get acclimated to a certain dosage of medication before adding a higher dose. I think its just less stressful on the fish.
As for the 75gallon... Definitely keep them out of light. I just want to make sure its definitely ich... there are other things that look white on a fishies body. Ich looks like salt sprinkled on your fish, and thats what you have... right??? You said there were two fish with ich... if you see a third fish come down with it, definitely use the quick cure, regardless if there is Nitrofura-G in there. If you are worried about med mixing.... remove the ones with ich and put them in isolation in a tank and treat with quick cure. The ich is more troublesome then the fin rot, so I would definitely keep a close eye on it.
If your med says specifically not to do a water change, then don't... but keep an eye on water parameters (ammonia and nitrite) and if they rise AT ALL... do a water change and just replace the meds you took out.... i.e. if you took at 10 gallons of water, at meds for 10 gallons when you replace. You MUST keep the water parameters good especially since you are overstocked (1 fish per 10 gallon rule) and another spike could prove fatal to your weakened fish.
(2nd message)
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2 of the goldfish are acting EXTREMELY weird. They sit at the bottom all the time, but when I went to feed them and when they went to swim around on occassion, they would dash across really quickly from one side to the other of the tank and do WEIRD sort of spins and somersaults like they were being dragged on a string.
This is called *flashin* and *scratching* basically your fish are irritated by ich (I am assuming they have it). Think of it like fleas on a dog... dogs can scratch, but fish can't. So they so all sorts of things to relieve the irritation. Some will scrape against gravel/plants, so will swim erratically, and others will sit in the filter outflow letting the running water sooth them.
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And another observation... the red streaks in the tails seem to be getting BRIGHTER and larger, almost like their spreading.
I know that the blue color makes it difficult to see the progress of the fish in the tank. I don't know if the red streaks are really getting larger or its just the color contrast that is giving that illusion. I just don't know... could you post pictures? The red should start to decrease soon. But the red streaks will take a few days to heal... don't expect that at the end of the treatment they will be all gone. As with any injury there is some healing. What you are looking for, in terms of a healed fish, is the loss of the whitish edge around the fins (as well as no more fraying/fin loss

) THATS what you are looking for. Once that goes away your fish will be on the road to recovery for fin rot.
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When I treated for ich before, it cleared up in 1-2 days, but this is taking a bit longer(which I was expecting naturally) but things seem to be getting gradually worse in the 75gallon and better in the 20gallon with bob, except for that blood streak prominence? Any ideas on the cause, or maybe I'm treating for something other than I should be?
I would check to make sure it is ich (salt grains). But if there are no more apparent external symptoms, and you have the tanks medicated, then there is nothing else to do but wait. A fishies natural reaction to stress (and the light being off) will be to sleep or rest on the bottom. The flashing is because of the parasite. and the fin rot is being treated, but it will take a few days to kick in fully. Bob is *probably* better because he is alone and not feeling the effects of ich and is an older, stronger fish in general.
I good way to peer in the the blue haze caused by the medicine is to grab a flashlight. DOn't shine it in their eyes cause it will freak them out, but you can spot ich and the fins with the light. Plus it will eliminate color contrasts from the blue so you can see the *real* status of your fishies fins.
Goodluck, hopefully everyone makes it through the night.