Decorate your aquarium without spending a fortune, and without stressing your fish. Avoid brightly coloured, mechanical, and light-up decorations found in pet stores that can be costly and would not be found in a natural fish habitat. Some of the best and most natural-looking decorations are also the least expensive.
Add rocks for little or no cost, but avoid those that might change the pH in an undesired way. Test this by placing the rock into a bucket with water from the tank. Test the pH of the water in the bucket, then retest it every day for several days to determine if there is any change.
River stones are a good choice for freshwater because their smooth round shape suggests moving water and anything that might leech out of the rock has already been washed away. Avoid rocks made of compacted mud and clay because they might dissolve and make the water murky. Be sure to sterilise them by soaking the rocks in bleach water for a few days, then soak them in clean water and aquarium dechlorinator before they're placed into your aquarium.
Aquarium plants are relatively inexpensive and easy to find. Purchase them at a local pet shop--a bunch should sell for under £1.30--or obtain free cuttings from other fish keepers at local aquarium club meetings. Plants such as Java moss or Java fern are easy to grow and will create an underwater jungle if given the proper care.
Unglazed Clay Pots
Add clay pots, available at your local garden store. They come in all sizes, from under an inch or less to very large. Bleach, rinse and dechlorinate the pots before putting them in the aquarium.
Fill them with substrate and plant aquarium plants; turn them upside down or half-bury them on their side to make a fish cave; or smash a pot with a hammer and use the shards as decorations. Caution: sand sharp edges so fish do not cut themselves when rubbing up against the shards.
Algae and other life will grow a patina over the surface of the pot over time, making it look naturalised in the setting. You can also root many aquatic mosses and ferns to them and they will slowly cover the pot.