Okay...the Bad News first.
- Your small tank has too many fish in it. Assuming that your tank is properly cycled, (which it won't be), a 10 gallon tank should only have a maximum of 10 guppies and nothing else.
- You've wasted limited money on buying plastic plants. Whilst these may look pretty, (although the Jury's out on that one ), they serve very little useful purpose.
- Your single Oto will be a miserable little fish and will be subsequently over-stressed. The Oto needs to be in a group of at least half a dozen. Those who know what they'r doing might keep a shoal of 10 - 15. Obviously, you don't have room in your tank for this species. I suspect you were sold it as an 'algae eater', to keep your algae down.
(Lordy, I'm growing to detest these stupid shops and their ignorant staff).
Now the (sort-of) Good News
We can help.
- Keep up with the water changes.
- Buy a test kit...you need to be able to monitor your ammonia, nitrite and nitrate levels. Liquid test kits are better, but for you, you just need something simple, because the only acceptable reading for ammonia and nitrites is zero.
- Swap the plastic plants with real ones. You don't need to be an aquarium gardener at this stage, just throw something relatively fast growing in there, like Elodea (pond weed). Floating plants are also recommended and these need no care whatsoever. These plants will reduce the toxic wastes in your tank, produced by the fish.
- Add some aquarium salt, 1 rounded tablespoon, per 5 gallons. Dissolve this salt in a jug of aquarium water, before adding it to the tank. The salt will boost the immune systems of the fish and help manage any bacteria and fungus and will help your fish repair their fins.
- Buy some bottled bacteria and add this to the tank. This should start your cycling process*.
*Cycling - a summary.
Your fish produce toxic waste, ammonia. With cycling, you grow beneficial bacteria which will turn that ammonia into nitrites.
Nitrites are also toxic, but you're also growing bacteria that will turn those nitrites into nitrates, which are far, far less toxic.
These bacteria can be added artificially, speeding up the process a little.
Live plants use ammonia when they're growing, go adding growing pants will significantly reduce ammonia and nitrite (and nitrate) levels.