Welcome to the addictive world of fishkeeping. I am so relieved you bought a larger tank. Goldfish grow large and are messy. They need a large tank and really good filtration. Comets are really more suited to a pond but a 230L is a good size.
1) The tank has one of those stick on thermometers. Yesterday and today (first couple days of being set up) the temperature has stayed between 21-22 degrees, even in the morning after having the light off for 9 hours. Are the motors in the filters heating the water up? Should gold fish be okay with this long term? Any predictions for what will happen in summer, when the days and nights are much hotter?
Goldfish overseas are raised in tropical temperatures so your fish will be fine. Their lifespans will be shorter though. If the tank has not been used for a while that stick on thermometer will probably be highly inaccurate. Where do you live? How high do the summer temperatures get? How hot does your house get? The motors in the filters, and lighting will heat the water a bit.
2) There are 5 fish in there (in a moment of weakness I ended up with another sarasa comet). I do feel a little guilty that I didn't cycle the tank without fish, and I know the more fish I add the tougher it will be on all the fish with more waste and whatnot. The two biggest are about an inch (excluding tail), the smallest ones a bit under that. They look ridiculous in the big tank, but my question is this: if I keep an eye on the ammonia, nitrite, nitrate and do regular water changes, will the fish be alright, for the most part? I'm growing attached to them, now.
The fish will be fine. They are very small so the tank will cycle with them in it. Do not do total water changes though. This will slow down the cycling. Keep an eye on the ammonia and nitrite levels. When these start to rise too much, do a 20 - 30% water change. With only 5 small fish in there you probably won't see much of a rise in these two things anyway (and a slight rise then drop is good as it shows the cycle is progressing) and the tank will cycle slowly and steadily with no harm to the fish
3) I've always loved plecos, and after the tank has cycled of course, is it possible to keep a bristlenose pleco with goldfish if I ensure the temperature stays around 21? Or will I end up with unhappy tankmates?
Check your tank is actually 21C first. It will settle at whatever the room temperature is and, depending on your location, this can drop quite a bit overnight. Bristlenoses are quite tolerant of cooler temperatures but they are not a cold water fish. I kept one with goldfish many years ago and it was very active. Did a great job cleaning up in the tank too.
4) I bought some oxygen weed and rubber-banded it to a log for the goldies to graze on. Are there any other plants suitable for goldfish aquariums? The plastic plants look rather silly in a big tank
Goldfish tend to be rather hard on plants. I collect oxygen weed from the local river so I don't have to keep buying it. Try Java fern or water sprite as well. The fish will nibble away at the bottom of the sprite but it tends to grow faster than they can eat it 8)
5) Will I be stunting them in this tank? Should they be okay for at least a little while, as they grow? The tank is 120cm long, so right now they have plenty of space to swim. I asked at the pet store, and the guy says that although they can't pay me for them, they do take back larger goldfish to sell on to people who (hopefully) have the right size aquarium or pond for them.
They will be OK a while as they grow but yes, they will be stunted, especially the comets. More though, comets are a short finned fish who like to swim. They can get up a good speed too so that is why a pond is a much better environment for them.
6) I'm thinking of getting a canister filter, but most of them seem quite expensive. Would it be a good upgrade if I had the spare cash? Are they any better than internal filters/is it worth it? What size or litre per hour turnover should I be looking at for my tank, and are there any that are cheaper but not cheap quality?
Externals are better and you won't have to clean them out as often. Goldfish produce a lot of waste and mess so you will be cleaning internal filters out a lot! Don't forget, all the good bacteria you have built up is in the filter media. Do not go replacing sponges. Wash them in some used tank water then replace them in the filters. Since you currently have 2 filters, stagger the cleaning regime so you are only ever cleaning one at a time.
There is great variance in turnover suggestions. One site says you need a water turnover of at least 6x the water volume per hour but the more the better. Another says 400L per hour. I can't do maths but I think there is a big difference between the two. You can't have too much filtration and the more you have with goldfish the better. There is a wide range of prices and the cheaper variety seem to run as well as the dearer ones, but probably don't last as long. I only have personal experience with Fluval, Jebo and a wet/dry Eheim. Eheim have a very good reputation but I would never buy one again as I have had nothing but trouble from the wet/dry. I gather that is the only filter in their range that had problems though and it has put me off them permanently. They are a lot more expensive than other brands but are a superior quality and last for years and years. Cheaper brands just last for years
