First you need to take a water sample to the pet store and have it tested. They will do this for free. Find out how good your water is.

I have found that goldfish if fed an all processed diet just always get rots, fuzzy bacteria, etc... By processed I mean the pellets and flakes. I have never had a goldfish live long on this kind of diet. They are prone to thses aliments. I add frozen to the diet as well as the flake and pellets. I put a piece of the frozen cube in a little cup and pour a little tank water over it, and then when it defrosts I use a wooden skewer to hand feed it to my fish. I give peas every other day ( without skin). I feed oranges fresh, or I will soak pellets in organic pure orange juice no calcium added. They sell a packet of frozen greens at the pet store for ominovious fish and they get this too. You have to build them up so when they are exposed to these aliments their own immune systems can take the offending bacteria out. I have never had a fish survive using the over counter medicines. The only exception the Jungle antibiotic food pellets they sell at the pet store. One thing that is wonderful about goldfish is they are not picky eaters. They will eat a lot of good things.

I have an indoor pond with tropical fish, two Koi, and two goldfish. The temp of the pond has been 60 degrees lowest, and highest 68 degrees this winter. I do not work heaters think nature even in the tropics has times of change. I know because the tropical orchids I have will not bloom till they get a room change that reads between 60 to 68 degrees. I keep my fish natural and a constant water temp is not natural. Also for me and my system there is always the risk of the heater burning through my preform pond , and some people have lost fish due to malfunctioning heaters. So far my tropicals have been very healthy. The Koi have been perfect. The goldfish this year got the fuzzy bateria on their skin. What I did was increased their orange intake , and fed them more greens, and the fuzzys went away in about three days by their own immune systems. They never acted sick or lost appetite. I just moved in with good food and let nature do its thing. I also keep my water very clean and fresh with regular water changes and real plants growing in the water.

sandy, what kind of frozen food are you referring to? My 5 large goldfish have been in a 100 gal stock tank for about a year --they'll go into big outdoor pond next yr.-- have being doing OK on different process foods rotated incl. spirulina. I tried feeding them peas once and they spit them out! They seem to prefer "junk food." Although the year they lived outside it was obvious how much they LOVED eating those bugs and worms.

Anyway, I'm sure there's truth in what you say-- it was amazing how much better my cat looks after starting her on a good feline diet-- shiny soft coat, etc. Only makes sense that fish too would respond to a good diet.

I think size of tank matters -- but how that might affect o.p.'s situation, I'm not really sure... 16 gals (63 l.) isn't tiny but its not really big either... I've heard 20 gals per goldfish is the recommendation for pond fish allowing them room to grow. The other thing I've been doing and which I consider vital for success is good filtration.

Good luck OP, hope you'll find the answer-- water test sounds like a good place to start.

PH of 8 is too high. Sandywesttexas is right have the water tested. PH and ammo is most important. Also measure your tapwater ph. Is the tank cycled?

The following foods I feed my fish. Only Hikari pellets (made by the Japanese. They use little additives and very little if any artificial colorings in their foods. Can be found at the pet store chains, PetSmart) . The flake I serve is made by ocean nutrition formulated for ocean and fresh water fish. It has the quality of frozen foods. I also use the Nutrafin Max flake food. What I would never buy is Wardley foods or tetra foods cheap dog food type food. I soak Hakari Algae wafers in a little tank water and then let them pull the pieces off the end of my bamboo wooden skewer.

The frozen foods I use are: Sally's Emerald Entree by San Francisco Bay company also mosquito larvae by the some company. Hakari frozen Bloodworms, Brine Shrimp, and Daphnia. I never just throw the frozen cubes in the tank I always put a little bit of the cube in a cup and let them defrost. Cubes can be easily cut with a sharp knife. So only use what you need. I feed my fish by hand. They have been conditioned to eat from a bamboo wooden skewer. You know the small kind they sell at the grocery stores. Just be careful not to skewer your fish. It works for me. Each fish gets a bite. I vary their foods. This afternoon I will be feeding them each a little mosquito larvae, a bite of flake, and a bite of pea. All at the same time. No fish in nature eats only one food at a time. They are little piggies. So I have always fed more than one food at each meal, but little bites. Do not over feed them. My fish are use to this and digest their foods well. I have never had a case of constipation. I think because with every meal they get fresh greens or fruit which adds that roughage they need.

James, a pH of 8 is not to high for goldfish. I have hard water here and the pH is usually around 8.2 and has not ever been a problem for my goldfish. I have never seen fin rot in my tanks. In fact I added two baby ryukins a short while ago, one had a big split in the dorsal...it healed completely in 3 days....I did nothing not even water changes in those 3 days. It was an established tank though.

I would recommend doing more water changes and definately as mentioned above...a varied diet. You could also add some aquarium salt to the tank, if you feel its necessary.

Good luck,
Sierra

pH is an issue to keep under watch (especially if your tank is crowded). It is measured on a scale of 1 to 14 to measure the acidity or alkalinity of your tank's water. Goldfish thrive at the 7.2 - 7.6 range (your pH testing kit will come with a colored chart to follow ) You will want to keep the pH levels in line or you could have dead fish. Goldfish are able to stand variations in pH much more so than other fish, but you should make an effort to maintain their water in this range. Over time, the water in your tank will become more acidic. Being aware of this and doing partial water changes is generally all that is needed to maintain the pH.

pH too low results in acidic conditions. This can be caused from too much fish waste in the water. (this is less likely to occur in a well-established tank) At pH levels below 7.2 Ammonia is ionized to Ammonium and is less toxic to the fish. Acidic water can cause your fish to weaken. It is best to do routine water changes to keep from having this type of problem. Symptoms: anorexia, excess slime, isolation, fish resting on the bottom of tank, and then finally blood streaking in the fins and death. (known as Acidosis)

pH too high results in alkaline conditions. Ammonia is more likely to be toxic when the pH levels are high. Symptoms: Fish will produce excess slime, and gasp at the surface. Losses can be severe. Once this takes place(Alkalosis) it is hard to reverse.

There are many brands and types of inexpensive tests available at pet stores to check your pH levels, as well as Ammonia levels. They have colored charts to follow for the diagnosis. There are products available to increase your pH levels as wells as lower them

This was taken from web site below

Here is a link that might be useful: goldfish

If the pH is maintained at 8.2 and does not fluctuate, the fish will be fine. I know this, because I have many years experience with this 8.2 pH.

The danger comes when inexperienced fish keepers try changing the pH to maintain a certain pH. Then the pH will rise and or drop....and this is what can cause major problems for the fish. Sometimes less is more for beginning fish keepers. The fish posted about in this post, already have serious health problems....they won't be able to take a rise or drop in pH, that can come from inexperience and dumping pH fixit in a bottle into the tank.

Also depending on what your local water is, depends on how it will react. I lived in an area with very soft water and had to watch because the pH would drop very fast. Here the water is quite a bit harder and the pH pretty much doesn't move.

Regular water changes are a must. If water changes ARE DONE on a regular basis, there should be no build up of ammonia either.

I recently lost a goldfish, that spent a couple days laying on the bottom of the tank.....I know for a fact that it had nothing to do with water problems......I tested and nothing had changed. Shortly after I added two ryukin fry to this tank....one had a split dorsal and it healed completely in 3 days. That tells me that the water perameters are acceptable. Plus these little guys are growing very quickly all in a pH of 8.2.

Sierra

Agree with Sierra, a sudden change in ph is usually fatal. Most fish can adapt over time to varying ph levels. I would check the stores ph also. At higher ph levels ammo becomes more toxic. If both fish in the tank are having problems it would point to a water quality problem. Again have the store check all water parameters, and get numbers not just good or no good.