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I wouldn't say yes you have to have them but, how can you not want a beautiful growing underwater garden with all it's different shades of green and reds and browns.
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due to growth and propogation your tank could be an everchanging paradise for your fish and a delight to soothe the eyes.
And then there are all the benefits that live plants bring,soaking up nitrates, heavy metals like copper and phosphates which cause algae. Some plants activly pump anti algae chemicals into the water.
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Lots of fish like to nibble on green plants,
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with a well planted tank and well maintained fish you don't have to worry about going away for a week leaving your much loved fish to starve 'cos they can always find something to eat
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(unless they are piranha fish).
Plants don't have to cost the earth either, buy them online in bulk and you can get about 90 plants for about £6-£7.
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Do what I do, buy economy collections and keep the plants that thrive, chuck out the one's that don't. If a plant likes your water, substrate and lighting conditions it will grow like wildfire, usually producing more plants for you at the same time