According to the test strip I have in ppm mg/L in the Tank:
Nitrate: 0
Nitrite: 0.5-1
Hardness 150 ppm
Chlorine 0
Moderate alkalinity
Neutral PH
This is why giving actual numbers for water parameters is important rather than saying everything is ideal/ good/ safe etc. There is actually a lot wrong about your water parameters.
I have pointed out the inaccuracy of your test kit, but let's take your parameters as they are.
The nitrogen cycle takes fish waste (ammonia) and processes it into nitrate, with a nitrite stage in the middle. Because fish waste turns into nitrate, you would expect to see zero ammonia and nitrite, and some nitrate in your water tests for a healthy established aquarium. Zero nitrate is ideal, but normally unachievable. So you want to keep nitrate below 40ppm, ideally below 20ppm.
In your specific circumstance, you have a couple of very messy fish that produce a lot of waste (ammonia), and all that ammonia should turn into a lot of nitrate. You are reporting zero nitrate, which tells me there is a problem with the accuracy of your testing, and the inaccuracy of your test kit has already been pointed out. And that inaccuracy could be masking problems. You are doing a quite big, frequent water change though, so that would help control nitrate levels, but it doesn't explain zero nitrate. If you had a handful of small fish in your 30 gallon aquarium, chock full of nutrient hungry plants, and you are doing big frequent water changes, then I would accept your zero nitrate. But a comet goldfish and pleco in a 30 gallon aquarium will be producing a lot of waste, and that should be showing up in your testing, but it isn't.
And then we get to the nitrite. In a cycled aquarium nitrite should be zero. Your nitrite is at a level where it will be causing issues with your fishes health. Nitrite poisoning causes a buildup of methemoglobin in the blood, hindering oxygen transport and potentially causing buoyancy issues. Which is what you are seeing. Now the fish has been moved to clean water the issue is starting to clear up.
The elevated nitrite could be down to your filter breaking down, or it could be down to your aquarium and filtration being too small for your fish and it simply being impossible to cycle the tank. Both are possible. Has the nitrite shown up in your testing since the filter broke down, or has it historically been there?
First up, you need an accurate set of water parameters, preferably taken before one of these big water changes. Either get a 2nd opinion from a fish store using a reliable test kit (not strips), or better still get a good test kit like the API Freshwater Master Testkit.
Should I return the goldfish to the tank now then?
Don't put the fantail back in the 30 gallon while there is nitrite still present. Fix the filtration. The fantail will need pristine water quality to recover. Once you are more confident your water parameters are safe, you can return the fantail, and then daily water changes for a while to ensure the fish has the best chance of recovery.
In an ideal world, the comet goes into a pond, the pleco goes into a suitable tropical aquarium, and then the fan tail can go into 30 gallon. The reduction of waste going into the aquarium might solve your water quality issue in there.
If you are planning on keeping the comet, the next best option is for the comet to get a bigger aquarium. A 50 gallon, with filtration rated for 100 gallons would be a better environment for your goldfish. Still rehome the pleco.
If you are set on keeping them both in the 30 gallon long term, fix the filtration and maybe a big, twice weekly 80% water change will compensate for the amount of waste your fish produces. Then the fantail can go back in the 30 with the comet. But its just kicking the can down the road and you will likely continue to see issues in the health of your fish, because 30 gallons isn't big enough for these fish to be kept together.
I think the pleco is a plecostomus
Pleco is just shorthand for plecostomus. There are 100s of species of plecostomus, and they come in all sizes. There are small plecos like a clown pleco, that gets to 3 or 4" and would suit your aquarium, to common plecos that get to 18 to 24" and need to be kept 100 to 200 gallon aquariums. While plecos are often kept with goldfish, they arent good tank mates as they have different needs.
Common plecos are often sold by pet stores as babies, to unsuspecting aquarium owners as a generic algae eater, with no consideration as to their long term wellbeing as adults. As the years go by and they grow, it becomes apparent that they need bigger and bigger aquariums. Unfortunately it's difficult to rehome them and a lot get dumped in ponds, lakes and rivers causing huge ecologically problems. Rehoming them is difficult because a lot of people are trying to get rid of them, and not many people want an adult common pleco. Even if they had a large enough aquarium for one, most people would have more interesting plans for it than a common pleco. The store you bought it from might take it off you if they have a conscience (they would probably take it round the back and euthanise it), you might have a local fish rescue service that would take it, you might get very lucky and find a hobbyist who actually wants one if you advertise. We have another member currently looking for someone to rehome hers.
My filter just stopped working today. We’re looking at ordering a new one online that arrives tmr, an aqua clear 50 hang on back style. Any advice on other filters that may work better ?
As said goldfish are very messy and need a lot of filtration. As a replacement I would look at canister filters rather than hang on backs as they will turn over a lot of water and have more capacity for filter media. And look for filtration rated for double your aquarium.
If you plan on keeping the 30 gallon, look for filtration rated for 60 gallons, so something like a fluval 307. I really don't think it will solve your issue long term, but might kick the can down the road.
If you can upgrade to 50 gallons, then look for filtration rated for 100 gallons, so something like a fluval 407. This will be more of a permanent solution, but ideally the comet should go outside in a pond. If you look to do this, make sure its an ornamental pond, not natural, and that you get the owners permission.
Take whatever filter media you can from your broken filter and use it in your new filter. This will help establish your cycle in your new filtration.
