tag... I'm it.
Small ornaments they can swim through also discourage following harassment.
I have a large ornamental oriental gate that both red and brownie like to hang around.
First I suggest a pre-filter sponge, they're sorta "cant go wrong" Fluval Edge ones will fit that filter's intake snugly, this will keep the girls from resting on the intake and possibly hurting themselves. They block material from getting into the fiberous filter cards and collect a great deal of solid non-decaying waste as well as keep leaves and girls from blocking the filter. In one case my experience has shown they're almost a must-have: Cambi a calico cambodian girl liked to sleep on my 150b's intake and one day I turned around to look in the tank and found her folded over around the intake strainer. She hadn't suffocated yet but her swim bladder was permanently collapsed and she spent the next seven months powering to the surface then gliding back down. Betta rely MOSTLY on their swim bladders for depth control. She was pretty, a spade tail and very strong. Miss her a great deal and if I ever find one like her such as Cammy or Clowntail I'll buy them even if I have to put them in their own tank. I couldn't breed her because of the damage, she'd have been very worth the try.
Java fern plants, http://www.liveaquaria.com/product/prod_display.cfm?c=747+892+800&pcatid=800
located in the rear corners is great, they don't need much light and they're very easy to care for AND generate new plants from their leaf tips. Cheap too.
While betta don't "need" water aeration I suggest getting a air pump and stone then routing the stone down in the back of the filter, even a little hawkeye will be enough. This is to support the biological filtration's needs.
Your bottom rock is a little thick/deep for unplanted, simple to solve with http://diszhal.info/english/plants/e...ne_wendtii.php
these like to spread large roots out and they're fairly resilient to simple gravel bottoms. Or you can get a couple water onion plants: http://animal-world.com/encyclo/fres...WaterOnion.php
this I've discovered produces a network of roots about seven inches in diameter and can solve the large open space problems while adding fair to good plant benefits to a tank. I've had one for a year and it has yet to give me any secondary plants. Mine gets lots of nutrition and sits dead center in the tank, their leaves are fairly stiff and will reach the whole length of the tank if you let them but you can thumb-nail pinch them off without the plant killing the whole leaf, letting you "groom" them to desired size. Since they will grow from the bottom glass to the surface of the water they're considered superior plants for biology support.
Anubias plants are nice as well but can be fairly fiddly and the girls often like to nose under plants for sleep, pulling them up out of the rock as they do.
Another thread here had a child's toy polyhedral mesh ball as an ornament in a tank and I'm sure the girls will love it as well.
Keep in mind that the filter you have may not be enough bio-filtration, you can supplement bio-filtration using sponge filters from http://www.jehmco.com/html/hydro-sponge_filters.html
these are carried by bigalsonline and several other stores, as additional bio support they're great and a good place to run aeration from as well.
If you put a pre-filter sponge on the filter's intake and run one of the hydro sponge as well you should have no problems biologically, once established such additions will likely be able to support the tank themselves. If you ever have long power outages the sponge filters can keep the tank alive using a squeeze bulb like comes with blood pressure test kits.
Keep in mind to place five girls in at one time and spend a large amount of time playing with them the first couple days. Sororities are best if you avoid duplicating colors (most girls will have a main, ray and sheen color, if all three match between an old one and a new one you're begging for brutality) and if you keep the tank well and it stays clean of infection it can be fun to put males in inside a 9-way breeder basket, a common thing in my house because it gets the girls going too. This is also a good tactic for healing a boy's growing fins.
IF you get a girl who likes to attack from below, take her out permanently.
IF you get a girl who likes to ram, take her out permanently.
If you get a girl who is over-aggressive, try letting her loose in a boy's tank for a minute or use training techniques such as a feeding time paddle (wooden spoon) to block her and protect younger girls.
I keep about eleven girls in my tank, names are a pain sometimes and on occasion all I can get as a replacement is blue ones.
If you have seven or fewer, which is advised in a 10, you will likely need to immediately purchase a new girl when one dies or the pecking order can be shaken up. When my alpha female dies I spend a whole day's free time asserting the new alpha female, I choose on personality, tenacity and experience in the tank but their aggressiveness isn't too important. Once they know YOU put them in charge everything will follow. You'll likely have to give them support (wooden spoon, net-time-outs at the surface) but the girls generally have a wonderful disposition towards each other once they become a school.
Crown tail girls are a hazard I hope you avoid, I'm thinking about setting up a crown-tail sorority but the kind of open feeding you can allow in a normal veil tail sorority will often result in crown tail girls becoming bloated and dying of distension and ruptures.
Feel free to pm any of the sorority keepers for advice on behavior control issues.