Trace ammonia, high nitrates in cycled tank?

[goldfish_is_orange]

My 20-gallon goldfish tank was fully cycled more than a month ago. After completing a week of 0 ammonia, 0 nitrite readings, I'm checking the parameters every 2-4 days.

Livestock
2 Orandas
1 Common pleco

Filter
I'm using a Seachem Tidal 35, with the included Matrix media.
I used to have a bag of Purigen inside, too, until the bag was torn while it was regenerating in bleach a week ago.
On the same day, I added 10 BOYU ceramic rings, to give my bacteria more room to grow. Basically to cover for the work that the Purigen used to do.

Plants and decor
Java Fern
Amazon Swords
Anubias Nana
Frogbits (which I regularly have to replenish haha)
2 unknown plants
Driftwood
A huge slab of manten stone
Lots of lava rock

Water parameters have been 0 - 0 - 10~20 consistently for the last month and a half.
Yesterday's readings:
Ammonia - 0.5
Nitrites - 0
Nitrates - between 40-80

I did a 50% water change with a double dose of Prime with the new water. I didn't feed the fish today. 24 hrs after the water change, I got:
Ammonia - 0-0.25 (not bright yellow, but also a tinge lighter than 0.25)
Nitrites - 0
Nitrates - closer to 80

Feeding is once a day. I'm alternating half a cube of freeze-dried brine shrimp, and vegetable pellets. I also give them the occasional broccoli or zucchini.

Where did the ammonia come from?? Was I overfeeding? Is it because I didn't have Purigen in there anymore?
Also, I would have expected the Nitrates to drop when I did the water change.

Do I need a bigger or 2nd filter? More filter media? Would a sand/gravel substrate help (to house some bacteria)?
I've only been in this hobby for 2 and a half month, so I would appreciate some insights on what's possibly happening here.

 

[Essjay]

Two goldfish and a common plec in 20 gallons is a huge bioload. It is possible that they make just too much ammonia for the bacteria and plants to cope with. The nitrate level does show that the plants are not using all the ammonia and there is plenty left for the bacteria to convert through nitrite to nitrate.
Adding extra filtration won't help if the current filtration is big enough to grow all the bacteria this huge bioload needs. Bacteria grow everywhere in the tank, not just in the filter.

The only realistic way to cope with these fish is a lot of water changes, possibly daily. You don't say how much nitrate is in your tap water but if that is low, you should do enough water changes to keep the nitrate level below 20 ppm.


You may not be aware but you will need a bigger tank soon. 20 gallons is suitable for just one goldfish so with two you will need to upgrade soon to at leat 30 gallons. And the plec will need a huge tank (over 150 gallons) as they grow to over 12 inches long, sometimes 24 inches.

 

[goldfish_is_orange]

Two goldfish and a common plec in 20 gallons is a huge bioload. It is possible that they make just too much ammonia for the bacteria and plants to cope with. The nitrate level does show that the plants are not using all the ammonia and there is plenty left for the bacteria to convert through nitrite to nitrate.
Adding extra filtration won't help if the current filtration is big enough to grow all the bacteria this huge bioload needs. Bacteria grow everywhere in the tank, not just in the filter.

The only realistic way to cope with these fish is a lot of water changes, possibly daily. You don't say how much nitrate is in your tap water but if that is low, you should do enough water changes to keep the nitrate level below 20 ppm.


You may not be aware but you will need a bigger tank soon. 20 gallons is suitable for just one goldfish so with two you will need to upgrade soon to at leat 30 gallons. And the plec will need a huge tank (over 150 gallons) as they grow to over 12 inches long, sometimes 24 inches.

I'll update you with the nitrate readings from the tap. I usually do 30-50% water changes every 4-5 days, and with the parameter readings I had for a few weeks, I thought it was under control.

I knew 2 weeks into the hobby that I would need to upgrade to a bigger tank, but didn't think it would be so soon. We had a plan to get an 80-gallon to house these fish, but that's put on hold because of reasons. We can't get a huge tank until Nov/Dec, so I guess I'll have to rehome the pleco now and see if we can manage with just the 2 goldies.

 

[Essjay]

Given the size tank the plec will need eventually, i would definitely try to rehome it. Two fancy goldies need a much smaller tank than a plec.