If you mean how does one lower the pH in water without using chemicals the only methods I know of are by adding Peat Moss to the filter and possibly hanging some in fine material bags (stockings) in the tank. Also using rain water rather than the reticulated tap water could help too.
Algae occurs naturally in any situation that has the three essentials for algal growth. They are Water, Light, and plant nutrients. If your tank has water and fish in it there will be food available so the only thing that may be needed is sufficient light.
It is possible, yes, but their chances for survival greatly decrease if they are not held separately.
It never hurts to have a filter it will only improve your fish's chances of survival and decrease your need to clean.
Salt Water Fish
It never hurts to have a filter it will only improve your fish's chances of survival and decrease your need to clean.
Assuming the dirty fish tank has had fish and water in it, without doing various specialised water tests it is impossible to tell what is in it.
It might be unsafe to drink because fish food, fish scales, fish waste, and many different kinds of bacteria are in fish tank water.
Live Aquaria provides you with supplies to preserve a fish tank with salt water fish. Fish tank provides lighting, fish food, and more to help you maintain your salt water fish tank.
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The white scale on the inside of the fish tank is from a buildup of minerals that were in the water.
Robert Frost
Scoop all your fishes to another tank or tub of water and clean the fish tank just as you would normally without the fish