it could be that within that 10 hours, your bio filter crashed. So the tank is needing to go through the cycling process to get the good bacteria back in order.

8 medium sized meaning 4-5 inches?? or 6-7 inches? Which is quite a load for a 35 gallon tank, and if in fact the filter crashed, it may be way the fish are lethargic.

I'd continue to do the water changes as you're doing, hoping to keep ammonia at bay while the filter re-establishes itself with good bacteria.

You may also want to add a bit of pond or aquarium salt to keep bacterial infections at bay while the tank is getting back in balance.

I agree. If you have the test kit check the ammonia level. Maybe do a substantial (50% water change) but be careful to replace the water with same temperature and dechlorinated water.

Then maybe 3 gallons a day for a week or more. Buy a test kit if you don't have one, then you can monitor the ammonia, nitrite, nitrate levels until they get back to zero.

an airstone and air pump might not hurt either. If anything it supposedly helps the beneficial bacteria established itself quicker.

Glen

Just chiming in with agreement regarding the chemical imbalance. Especially because 8 goldfish is a very heavy load on a 35 gallon tank. Usually it is recommnded that each goldfish have ten gallons, minimum, so ideally your tank would only have three fish.

Just to get everone informed. These fish have been in this tank for well over three years. The largest fish (body-not counting tail) is 3 1/2 inches. Long tails on all fish. Real plump, healthy fish until this. I have an undergravel filter with a power head, also last month added a Whisper Power filter to the tank due to the fish growing a bit. I just tested the water- 0 ammonia levels and slightly alkaline. Two of the fish are starting to get "blood streaks" in their tails. They are still going to the bottom of the tank and "crowding" together. One is starting to lose balance. I'm changing 4 gallons of water today. I'm really getting frustrated....I don't want to lose them.

The blood streaks are due to hemorrhaging. You should measure the nitrite and nitrate levels in your tank as well as ammonia, if your tank is re-cycling then ammonia could be zero but nitrites high (>10-20 ppm would be considered high). This creates a toxic environment for the fish. If the problem is water parameters you should consider only feeding your fish every 3-4 days until you have it under control. This will cut down on the amount of waste in the tank.

I've also heard that in cases where the power is off for several hours and the tank has an undergravel filter, the toxins being held under the gravel by the pump will return to the main part of the tank and can be upsetting to the fish, acting almost like a poison. It can be really bad if the contents of the undergravel filter have never been removed (I think it's recommended that the undergravel filter be siphoned out every few months). If that's what happened then you should do frequent water changes of 25% until your fish improve. Some people avoid the undergravel filter altogether for this reason.

I do not doubt your goldfish have been fine for a while, all the more reason why a sudden change can be stressful to deal with.

With a heavy load like that when you lost your biofilter you are going to go through a bad cycle. Now enough bacteria has to grow to support 8 fish in 35 gallons. Get a nitrite nitrate test kit and use it every day. You mentioned the ammonia reading was zero that is good maybe you did not lose much of your ammonia reducing bacteria but it is the nitrite bacteria that you do have to worry about. It can kill just like ammonia. You may even have to do fifty percent water changes daily for awhile to keep your fish from crashing. They are crashing right now feeling the high nitrites. I just started a 125 gallon indoor pond and I used stability bacteria and a living nitrite distroying bacteria found in the refrigerator of your pet store. Sorry can not remember its name.Place all the bacteria you buy into the filter directly not the water you want to seed your filter and increase the air supply in the tank. Unfortunatly it is the nitrite eating bacteria that grows so slow and is so fragile. Very dependant on air. It took twenty days to cycle my pond, and I added some living matter from another tank I had. Durning the time my nitrites went to 3 to 4 ppm I was doing a few 50 % water changes and always every day changes which will slow down your cycle but your fish will live. Add Aquarium salt three tablespoons for every 5 gallons. Get all plants out of the tank salt will kill your plants. Do not medicate your fish it will kill off your biofilter. salt is the best medication you could give your fish that and good water should heal fish and will keep them from suffering from the nitites. Increase salt slowly you will want to add only one tablespoon for five gallons wait a day then add another tablespoon for five gallons and wait a day then add another tablespoon for five gallons till your have a total of three tablespoons for five gallons of water . You are increasing the salt slowly so as to not shock your fish. this dosage will not harm your fish it is the 3% they use when fish are very sick. I used 3% in my pond durning my cycle. Keep in mind I got a high nitrite reading durning my cycle just by having three 4 inch goldfish in 125 gallons. You are going to go through a terrible cycle due to fish overload. I lost one of my three ( a blue lionhead) due to nitrites Not even the salt protected him some fish are more susceptible then others. He was having rapid gill movement and sitting on bottom of pond. Professional goldfish breeders say that 30 gallons of water for each adult goldfish is not tooo much . That is why right now I have three goldfish 4 inches each in my 125 gallon pond and that is all I intend to have. They will get big have plenty of room to grow and I should not have any problems with water chemicals. Remember every time you do a water change you will have to add back the salt you are taking out say you add five gallons to the tank then you will need to add back three tablespoons of salt. Salt does not evaporate out of the tank so only water changes remove it. Measure it out exactly too much salt will kill your fish. 3% salt is very healing... I do not fear salt as much as all the over the counter fish medicines they often kill the fish and in terrible ways. The salt also will kill any parasites too.

3 1/2 inches after 3 years is not very big. They should be much bigger. Get a bigger tank! It will be much easier to keep them healthy if you do.

Also, the average lifespan for a goldfish is 20-25 years, so three years is really not that long....

I think melle hit it on the head. In an undergravel filter the nitrifying bacteria is kept wet (gravel and build up under the filter plate). They should be taken down and cleaned every 6 months. The anaerobic bacteria (build up under the plates) and other crud could have softened and fell off after the blackout since it was extended. When the power kicked back in and waterflow pushed the crud up it could have caused what sounds like a bacterial disease.I would tear down the tank, scrub the plates(actually I would toss the UGF totally and go with a good HOB or canister filter)and place evry thing back with 50% pwc every other day. A micron filter or purigen bag would help Polish the water. If you are useing powerheads you can get or make a polishing filter for the intake. Also as the fish get bigger the 35 gal will be to small for eight. You may want to look into a 55gal or another 35 gal and seperate them so they can grow into the tank. If you dont then you will be looking at a future problem especially with UGF it is disease waiting to happen. Good luck.