Things can look cluttered in one layout, and look just right in another.
Here are a few tips:
Use more of the same thing to make a larger mass, but still not look cluttered. Several plants of the same species to fill one corner, then several of another grouped together in the other corner. Perhaps one is a cluster of grassy looking plants (Vals, Sag...) and the other is leafier plants (Swords, Anubias, Java Fern) Choose one species of ground cover plants.
Cluster things. An arching, branching piece of driftwood can have some plants growing up between the twigs.
3 to 5 rocks (all the same sort) can be in a pile, loose stacked, and the Loaches will love it. A plant that is the same as has already been used can be planted right behind this pile. Or choose a red leafed plant, or something different to accent the pile of rocks.
More cluttered look:
One of these, and one of those, and one of something else...
One over here, one over there, nothing touching...
If it is fish, then 2 of each species makes a 'Noah's Ark' tank. Does not look natural.
I would eliminate most of these. In a 100 gallon tank, get 2 schools of a dozen or more reasonably small fish. Pick one upper level and one mid level species. (No more bottom level fish)
6 rasbora
6 black tetera
6 bunes areis
12 other fish
Layout:
Mentally divide the tank into 1/5ths Put one of the biggest showiest things at the 2/5 from the left. Put another showy something that is a little taller or a little shorter/wider at 2/5 from the right.
Fill in with smaller things so the feature items are not isolated. Ground cover and low growing upright plants, a gravel trail, Similar collections of rocks.
Google Amano for some really incredible aquarium layouts using rocks and plants.
38 tanks, 2 ponds over 4000 liters of water to keep clean and fresh.
Happy fish keeping!
