What could be wrong with my dalmation molly

[Essjay]

And what does soft water mean

Water contains minerals dissolved from the rock it passes over after it fell as rain. Some rocks, such as limestone and chalk, dissolve a lot and the water has a lot of calcium and magnesium dissolved in it - this is called hard water. Other rocks hardly dissolve at all so there is very little calcium and magnesium in it - this is called soft water.

If fish have evolved in hard water, their bodies can deal with lots of minerals but they suffer mineral depletion if kept in soft water. Fish which evolved in soft water get calcium deposits in their kidneys if kept in hard water.

Mollies are hard water fish which need a GH of at least 14 dH (or 250 ppm) and they are prone to disease if they are kept in water with a GH lower than that.

 

[Christina88]

Hey, it's great that you're looking to upgrade the tank size so you can keep the fish you have healthy
Yes, it sounds like a 30-40 gallon will be big enough to house your pleco as an adult, you'll want to upgrade pretty soon as they grow quite fast, and you want them to be in the larger tank before their growth is stunted.

Having more water volume will definitely help you keep water parameters more stable, just be the nature of dilution. A small tank with a heavy bioload, like a ten gallon with a pleco - gets toxic much faster than a 30-40 gallon would. The live plants will also help. But the main way we reduce nitrates in a tank is through water changes. Up the water changes to twice a week, and you should see a big improvement until you upgrade to a larger tank. Then once a week to maintain should be enough

Some more info about the nitrogen cycle, it's important to understand what cycling is in order to understand how to manage ammonia, nitrites and nitrates, found this video very helpful for explaining the nitrogen cycle clearly:

And more info about your species of pleco (thank you for the ID, @Bub !)

Oh, and you need to get some driftwood for the tank as soon as you can, even a small piece will do. Plecos need to be able to rasp on some driftwood to help their digestion, so that's an essential even before upgrading the tank.

Panaqolus sp. – L403 (L403) — Seriously Fish

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Ok I'll get some wood and thanks so much for all the help. I didn't even know that my Pleco would be big or needed wood. Cause I asked the fish store about it and even said I only had a 10 gallon tank but he said it wouldn't grow any bigger and it didn't need anything special to keep it happy and healthy. Wow I really need to stop learning to fish store and just come ask ya'll. Cause I've been told wrong this whole time and I did some research and watched a lot of videos but it didn't help much as I thought.

 

[Christina88]

Water contains minerals dissolved from the rock it passes over after it fell as rain. Some rocks, such as limestone and chalk, dissolve a lot and the water has a lot of calcium and magnesium dissolved in it - this is called hard water. Other rocks hardly dissolve at all so there is very little calcium and magnesium in it - this is called soft water.

If fish have evolved in hard water, their bodies can deal with lots of minerals but they suffer mineral depletion if kept in soft water. Fish which evolved in soft water get calcium deposits in their kidneys if kept in hard water.

Mollies are hard water fish which need a GH of at least 14 dH (or 250 ppm) and they are prone to disease if they are kept in water with a GH lower than that.

Ok so how do I get the water as hard water? And sorry for all the questions but I really love my fish and want to do what's best for them

 

[Essjay]

The way to make water harder is to add more of the hardness minerals. Something like Seachem Replenish or with mollies even Rift Lake cichlid salts would be OK. You would use these by adding them to the new water at a water change before adding it to the tank; and the amount per gallon of water should be the same every time to keep the GH in the tank stable.