Switching gears to goldfish?

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Hello! I posted a couple of days ago how I had a 30 gallon tank with five tiger barbs + 1 mystery snail. I am considering taking the Tiger barbs and putting in 1-2 fancy goldfish. I had a couple of questions specific to goldfish before I make any decisions.
  • How many goldfish could live happy in 30 gallons?
  • I have a substrate of 20 lbs sand and 5 lbs gravel (give or take a bit from time). Do you think they could come on gravel? And if yes, could I top it off with another form of substrate to cap it off and make it doable?
  • is there a recommended species of goldfish for 30G? (Oranda, telescope, ranchu, lion head, etc.)
  • Other than water quality being important , are there any things that are very important to now about goldfish?
  • When switching from tiger barbs to goldfish, other then temperature what else would I need to change? Would I need to re-cycle?
Thank you for taking the time to read this! Any advice is appreciate!!!!
 
So, to be honest I would not put any goldfish in a tank that size. Many sites say 20 gallons for one goldfish, and 10 for each additional but I would say at least 20 gallons per fancy goldfish (not including the leaner bodied goldies like comet, common and shubunkin which are pond fish).
Fancy goldfish are commonly prone to a variety of ailments such as swimming issues (either temporary or permanent), ich and fungal infections; based on my experiences with them.
Additionally fancy goldfish have compact bodies with short digestive systems, well not realy a system but more like a straight line of in-out with little digestion actually happening...they are very inefficient in nutrient absorption and that is because we bred them to have these cute compact bodies. This doesn't mean they they don't grow to be large fish, they can get to be 8-15" depending on how old they are.
Due to their digestive systems, they need frequent small feedings during a day; 2-3 times at least. This also helps to foul water fast. So to help with keeping the water clean a filtration of at least 10 times the tank volume should be installed. So for a 30 gallon tank you would need something that actually (advertised gph aren't always true) filters 300 gph (or multiple somethings that total that amount). Also goldfish like to forage so they can and will swallow substrate and if it is too large they will choke on it so gravel isn't preferred unless it is very small. Sand is better, however since they like to forage they can easily bury food and waste in the sand, causing issues in the long term (as sand is hard to clean without vacuuming it up with the waste and water. You can add trumpet snails to help clean up the sand, but that can be an issue as goldfish like to eat snails too...ask me how I know :rolleyes:.
Goldfish will uproot and eat anything planted in the tank, also very few plants can be found that like the colder waters that the goldfish need. Some floating plants like anacharis/elodea and hornwort are quickly consumed-even when planted. Other plants that are touted to be for coldwater like anubias and java fern, don't do well in my experience. You can go with plastic of silk plants but that can and will get in the way of the bumbling fancy goldfish and are a no for telescoping eye goldfish that can have issues with vision as well as swimming.

If you insist on turning the tank into a home for a single goldfish then I would recommend redoing the substrate completely for sand. Doesn't have to be a thick layer, or you can go bare bottomed. Also still boost the filtration so it is at least 10x the tank volume. Add another filter or get a bigger one and transport the old filter media into the new filter to keep the cycle. Keep the new media as well as the bacteria will grow to fit the needs of the tank and the new media will give added space for the bacteria.

Hope this helps you make your decision.
 

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