Sad Molly ?

In one of my tanks, I had (among the other fish, a combination of mollies, guppies, angels, and zebra glofish, and a golden Chinese algae eater), two white mollies - a male and a female - which I purchased together and have always swam together in the tank.

The tank is overstocked for its size, but the parameters are all normal
7.2pH
<0.1 ammonia (the API test kit does not quite show the bright yellow it should, but comes nowhere near the color of the next marking)
0 nitrite
<10ppm nitrate
8' GH
6' KH
79'F
I clean the tank and do 30-50% water change weekly

Last week, the male molly died, fairly suddenly. Ever since, the female has been swimming with clenched fins.
Could this be a sign of disease (all the other fish seem fine), or could this be something "emotional"?

The problem is the hardness of your water. Mollies are hard water fish which must have a hardness over 15 dH. Your water hardness at dH 8 is too soft for them. When the water is not hard enough, they suffer calcium depletion and become sick easily.
The other fish in the tank are soft water fish, which is why they are all fine.



Keep an eye on the Chinese algae eater. As they mature they develop a liking for the slime coat of other fish, and deep bodied fish like angels are particularly at risk.

I know about the algae eater.

I always heard 8-11' for mollies, guppies, etc. In API's test kits, they say 6-11.
What struck me as strange here was that my other mollies are all totally fine.

Seriously Fish (the one website you can trust) gives the range for mollies as 15 to 30 dH.

How long have you had the mollies?
It could be that the one which died was weaker than the others for some reason, and it succumbed to the low GH quicker.

I had that one for about three months.
I bought 2 others at the same time, and 2 more recently.

For some time, the gh was higher - around 14-15' - and over the last few weeks, I intentionally - gradually - lowered it, based on the information from API

3 months is a short time; the other mollies will be suffering long term damage, unfortunately. So will the guppies, though they need water slightly less hard than mollies. I'm afraid the API instructions are not accurate.
But the angelfish won't be able to cope if you allow the water to return to it's normal hardness.

This why we always say that hard water fish and soft water fish should not be kept in the same water.

Ok.
Thank you for you help.

Since my original message, I've been loosing two fish a week, each with different symptoms.
The strangest was my Chinese Algae Eater, which developed this over about two days:



Parameters:
7.4pH
0 ammonia (the API test kit does not quite show the bright yellow it should, but comes nowhere near the color of the next marking)
0 nitrite
25ppm nitrate
14' GH
80'F
I do 30-50% water change weekly (I assume that's what raised the pH and GH, since the water here is naturally hard, and I've stopped filtering it)

I have never had problems in this tank before, and can't figure out what's happening.
Any advice would be greatly appreciated.
Thanks

The algae eater has an internal bacterial infection.

Do a 75% water change and gravel clean the substrate any time a fish dies in the tank. You should also do a 75% water change each week. Small water changes leave lots of gunk behind, big water changes leave less gunk behind.
Make sure any new water is free of chlorine/ chloramine before it's added to the tank.

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You can add rock salt (often sold as aquarium salt), sea salt or swimming pool salt to the aquarium at the dose rate of 2 heaped tablespoon per 20 litres of water.

If you only have livebearers (guppies, platies, swordtails, mollies), goldfish or rainbowfish in the tank you can double that dose rate, so you would add 4 heaped tablespoons per 20 litres.

Keep the salt level like this for at least 2 weeks but no longer than 4 weeks otherwise kidney damage can occur. Kidney damage is more likely to occur in fish from soft water (tetras, Corydoras, angelfish, gouramis, loaches) that are exposed to high levels of salt for an extended period of time, and is not an issue with livebearers, rainbowfish or other salt tolerant species.

The salt will not affect the beneficial filter bacteria but the higher dose rate will affect some plants. The lower dose rate will not affect plants.

After you use salt and the fish have recovered, you do a 10% water change each day for a week using only fresh water that has been dechlorinated. Then do a 20% water change each day for a week. Then you can do bigger water changes after that. This dilutes the salt out of the tank slowly so it doesn't harm the fish.

If you do water changes while using salt, you need to treat the new water with salt before adding it to the tank. This will keep the salt level stable in the tank and minimise stress on the fish.

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If you can post pictures of the remaining fish and maybe a short video, it might help us identify the problem.

The following link has information about what to do if your fish get sick. It's long and boring but worth a read when you have spare time.

Sorry about your fish

I did a 75% water change using (as I always do) dechlorinated tap water, and, according to Collin's instructions, and added 2T salt per 20L water.
I did my best top maintain the temprature and ph of the water used for the change as well.
40 hours later, the parameters are still fairly constant:
Ammonia 0
Nitrite 0
Nitrate 5
ph 7.4
kh 6-7'
gh 8' - regarding this: I have regularly checked the gh of both my tank and my tap water over the last six months, and found both to fluctuate, generally, but not always, in sync with each other. I know that my regional water company has several sources they draw on depending on the season, including lakes, runoff, and aquifers, and I suspect some of these sources contain different minerals which are not filtered out. While it is well known that the local water is "hard," it seems the actual hardness varies, presumably based on the source.

I have read that post on sick fish, and followed most of its instructions a soon as I noticed something was wrong. My problem was that I continued to repeatedly follow those instructions, rather than seeking help, and have lost, as I said, about 2 fish each week for almost a month.

2 more of my fish - a zebra glofish and a guppy - have died since the water change.
I realized recently that all of these issues started when I was away and missed a weekly water change about 4 weeks ago. My other family members are more than happy to feed the fish, but not to do other maintenance.

I have also noticed that different species that are in the tank seem to exhibit different symptoms of stress or ill health before their deaths.
My angel that died just seemed lethargic for a few days.
The mollies also seemed lethargic, ans swam with clenched fins.
The guppies showed no syptoms that I noticed.
You saw the picture of the algae eater.
The zebras appeared emaciated, but not lethargic at all.

One of my zebras:


General view of the tank:





The reason I removed the decor
This tank was not planted, but did have a few fake plants and some rocks. While cleaning the tank this time, I noticed that there was some sort of black sludge in the crevices between the leaves. I don't know if this is just a build up of dirt, or something more sinister. I have never seen it before. Regardless, I removed the plants to thoroughly clean them.

I will leave those more experienced with diseases to comment on those. But fwiw a gh of 8 degrees is not hard in fish keeping terms and is too soft for mollies and guppies. In the long term they will have health issues.

Also fwiw when I go away I hide the fish food and instruct my family not to feed my fish. Overfeeding does far more harm than not feeding at all.