Ok definitely listen to the advice about getting the API master test kit. Life saver! Also if you notice the fish is starting to get sick I would quarantine that fish! Don't let your other fish peck or feed of their body. How much and how often are you feeding them? The floating and then sinking symptoms sounds like swimmers belly. When they sink and can't reach the top they can't get oxygen. If this may be a problem try sticking them in a tank with only a few inches of water so they can reach the top easier and don't feed them for a few days. Overfeeding can spike your ammonia levels in the water quickly, which is another reason you should test often. You do not have to feed your fish everyday. I like to feed mine every other day and then every once in a while I wait two days to feed my fish. Also do your water changes! Especially if your water parameters are an issue. Once you get the under control, water changes weekly. Depending on how many fish are in the tank and how your parameters are, water changes of 25% to 50%. If you have aarge spike 75%. I typically do a 50% water change but it depends on your tank! I hope this helps!
I have to say I think your problem is way too many fish in the tank which is contributing to the ammonia spikes. The rule of thumb is 1 inch per gallon. So mollies are about 2-3” & guppies 1-2” for instance but dwarf gourami & plecos are much bigger. For a 25 gal tank & your variety you should only have about 8-10 fish total. Pleco is probably too big. Also you should only have 1 dwarf gourami. They Can be aggressive & can kill your other gourami fish. 1 male per tank especially that size tank. Also, how much do you feed the fish? Some of the symptoms appear to be swim bladder so possibly too much food. I would recommend a bigger tank or another tank to split the fish up. I would see if you can return all the gourami you just Bought.
Since the start of this year I have been keeping fish. First I got a 110 litre tank (around 25 gallons) and I have had so many problems. I have had a few ammonia spikes and lost quite a few fish to that. The most recent spike was about two months ago and I have lost a few fish because of that. I get the ammonia and nitrate tests done at my lfs so I'm not sure how big the spike was. I have also got a 22 litre tank (about 5 gallons) as my betta was attacking the other fish but that is sorted now and I have no more issues with that. However, recently my fish keep dying in the 110 litre tank. Yesterday I had the first death for two weeks and talked to my lfs about this but he said it could just be a one off from old age. The deaths before yesterday weren't very often but they all died in the same way: they start floating around, swimming weakly, falling to the floor, rapidly breathing and then dying. The other fish in the tank peck at whatever fish is dying but don't fully eat them and there were no chunks. About 3 weeks ago, one of my corys died which I assumed was because of ammonia poisnoning except I couldn't find its dead body in the tank. I was only gone for about half an hour from the point where he was almost dead and when I came back he was gone. At the time, I took out all of the decorations in the tank and couldn't find any signs of the dead fish anywhere. Today I had been gone all day and came back home to find my pleco missing. I can't really remember seeing him yesterday but I think I would've noticed if the fish were all in one area picking at the ill fish. I have no idea what they died of and I have no clue what I can do next to make sure no more die. Anyone have any ideas?
The fish in my 110 litre tank: 2 baby balloon mollies, 3 endlers, 3 guppies, 5 swordtails, 3 balloon mollies, 1 cory, 4 dwarf gouramis (I got these recently before I found out the other fish died), 1 zebra pleco, 1 common longfin pleco (this is the recent one that might be dead).
Water Parameters: The ammonia and nitrate is 0 (tested today) but not sure on anything else. Temperature is 25 degrees C
Water Changes: About 2 months ago my lfs recommended to stop doing water changes but I did a 15% water change 4 weeks ago and topped up the water in the tank 2 weeks ago.
Chemicals and treatments: I add bacteria pearls every week, tap water conditioner to any water added, and aquatic plant food
New fish: 4 gouramis but after the fish died
I will add photos off the tank set up (the net holds the molly babies)
I believe the tank hasn't fully cycled. If fish die after a water change that would mean either you are not using a chlorine rid or tank water not cycled. Which you are using I recommend buying a refrigerated bottle of water cycle. that jump starts the cycle. Don't buy any fish. than buy an API Master Water Test Kit. Take readings everyday. Don't change the water until you see O ammonia,nitrites and nitrate. You might loose more fish but using the water cycle bottle should stop that. You have plants. How are they doing? Are they green or are they turning brown?
I do the old fashioned method of letting water sit for at least 3 days to allow the amonia to evaporate. I don't know if it also allows other chemicals to evaporate too. Other things to keep in mind: 1. when buying fish, look at all the fish in the tank you want to purchase from. If the fins look deformed or it there are dead fish in the tank, do not get fish from that tank at that time. 2. Keep the tank at the lower end of the acceptable range of tempurature. I made the mistake when I was young of thinking, well they are tropical fish, they like it warm. The range was 70-80 degrees F, so I kept it closer to 80. Wrong, more disease happens when it's warmer. 3. Study the fins and bodies of the fish that are sick in your, then look up pictures of fish diseases and see if you can identify if it's a disease process. Fin and tail rot and Ick are real common. 4. Do you have an under-the-gravel system? They can help and there are either charcoal or amonia cartridges that can be installed to help with those areas. (Note: If you do not have this, only try to install one as a last ditch effort as this will require basically starting over.) 5. Finally there is a maximum number of fish you can have in any tank based on the size of the fish and the number of gallons to your tank, as someone else mentioned. It is 1 inch / gallon. So with a 25 gallon tank you can have 25" of fish, stretch end to end. If the fish are 3" long, that means 8 fish. If they are 2" long that means 12 fish. Good luck. I'm not the greatest caretaker of fish, but these are a few more basic things that can help in the beginning of having a fish tank.