fin rot and red/brown bump

so tonight I was observing my fish and noticed a red/brown lump on my cobra puppy's dorsal fin. I then checked out all the other fish and found no lumps but found some other fin problems. one of the harlequin raspboras is missing the top of his tail fin. one sterbai cory has damage in tail fin as well as shriveled up side fins. everyone else looks ok? 25% water change tonight and replaced oldest filter.

Tank size: 20ga
pH: 7
nitrite N02: 0.5
nitrate N03: 20
General hardiness GH: 60
Carbonate hardness KH: 40
tank temp: 74
Filtration: 2 top fin 20s
Current tank inhabitants: 3 neon tetras, 1 cobra guppy, 3 harlequin raspbora, 2 sterbai cory and 1 julii cory

updated photos, the one sterbai does have a brownish growth potentially on the curled/clamped fin

2 more images... any help would be appreciated! going to do another 25% water change tomorrow

Hi,
Sorry you have not got any answers yet. Hope someone can help soon. Maybe some more info would help.
Can you get a clearer photo of the cory?
How long has the tank been running?
Do you normally have a nitrite reading?
Can you test for ammonia?
How do you clean the filter?
How much/ how often do you do routine water changes?
One problem I can see for your guppy is the soft water, it needs 200ppm or higher.
But there is something else going on in this tank, @Deanasue or @Colin may be able to help, especially if you can give some of the info above.

thanks for your reply. attached are a few more photos. he is still pretty agile but his left fin is like a little brown club. very hard to photograph but it looks like I might fall off entirely.

tank has been running a few years. lost a raspbora in june, 2 neons in april, 1 neon july 2018 and so on... no additions to tank in over a year. the guppy is actually 3 years old I believe? he has survived many tragedies.

chemisty: what I posted for chemistry is what I have for test strips, you asked about nitrite level is typically 0 to 0.5 in my notebook. also the hardiness levels have stayed about the same for as long as I've been recording.

cleaning routine: I do water changes every 3 to 4 weeks, usually 25-50% depending on brown algae buildup. I always replace my oldest filter and cold water rinse the fresh charcoal. the other filter I rinse with tap water on full pressure from exit side of filter so stuff falls off the entrance side of filter. that then drains/drips out as I am paper towel wiping the walls, or washing decorations in a hot water rinse. decorations usually only get cleaned every other time or again depending on brown algae levels. I always treat the water with 5ml per 5ga with stress coat. it sits 15-30 minutes before adding to tank. I then add bacteria suppliment straight to tank.

I know that's a lot of info but hopefully helps guide someone to an answer of what is wrong with these little fishes.

one more comment, last week I noticed the temp had spiked to and held at 81/82, not sure for how long. I corrected the temp but hadn't looked close enough as everyone was swimming well and came out for food. not sure how long it was elevated or if it can cause this

One thing that stands out to me is your tank maintenance regime. You should be doing weekly water changes of at least 50%. The gravel should be cleaned by pushing the siphon tube into it, avoiding the roots of any live plants. Filter media should be cleaned about every 2 weeks by washing the media in old tank water not under the tap.

What is the filter media you change? Sponges and ceramic media will last for years and only need regular washing. Carbon cartridges can also be washed, but a better way to deal with these is to cut them open, remove the carbon, which is not needed on a routine basis, put the empty cartridge back as it will have bacteria on the fabric and place some sponge in the space created by removing the carbon.

Since you use test strips, there is no ammonia reading. There should not be any in a tank that has been running as long as yours, but it is worth checking.

The brown algae you report needs to be investigated. Something is out of balance if it grows so much that you have to clean the decor every few weeks. This, in addition to the tank maintenance, could be affecting the fish.
Do you have live plants, and if you do, how much and what plant fertiliser do you use?
How long are the tank lights on for?
How much do you feed the fish?

Hi! I think your biggest issue is water quality. Essjay has covered that very well above. If you only change your tank water and clean every 2 weeks then your fish are living in a lot of bacteria. This in itself can case numerous issues. For now, I would do big water changes every couple of days for a couple of weeks. See if that helps clear up the issue. Brown algae can be from too many nutrients in the water. Do you have a lot of plants? They really help. Shorten the time you leave your lights on and see if that helps. Also cut back on the amount of food you are feeding. I would normally say to add dissolved aquarium salt but the bottom feeders don’t like salt. Try the water changes and other suggestions and let’s see how it goes. If no improvement in a week or things worsen, then we will talk about meds to use. I think the clean water will do it.. Monitor your parameters for ammonia, nitrite, nitrate. I would suggest buying an API Marine Freshwater Testing kit. Keep us posted!

Deanasue said:

I would suggest buying an API Marine Freshwater Testing kit

Click to expand...

Slight typo, Deanasue! It's the API Master Freshwater Testing kit

Just so we don't get anyone confused with the test kits meant for saltwater tanks.

essjay said:

Slight typo, Deanasue! It's the API Master Freshwater Testing kit

Just so we don't get anyone confused with the test kits meant for saltwater tanks.

Click to expand...

Oops, thanks so much for catching that, essjay!

thanks for the replies. I will work on my cleaning schedule. I will do 3 water changes a week for a couple of weeks and keep a close eye.

there are no live plants in the tank

tank light are on 6pm to 11pm

feeding is about five nights a week as I miss a few here and there. they get 2-3 shrimp pellets, and every few feedings a small small amount of flakes that I finely mush in palm and release below the surface of the water.

ok so I have been doing my water changes every couple of days. 25% at a time. the cobra guppy is getting extremely fat. I moved him to a breeder tank tonight so that he doesn't eat with the others.

the rasbora with the damaged fin now has alot red near where the skin is.

the sterbai cory still has clumps on his fins, they almost look like they have gel on them. in the "armpit" of the fins it looks like there are new fins growing.

3 very different things occurring and dont know where to begin.

more pics of the red, fyi I think the cobra guppy was biting him

Doing 25% water change really isn’t enough. It leaves 75% of the bad water in. Do at least 50% but preferably 75%.