Well, first make sure she's not eating rice cereal. Rice cereal is a natural constipation causing food. So are soybeans (if you have her on soy formula or have been feeding her tofu or something). Bananas can also cause some people to firm up, so if she's eating a lot of bananas you might want to ease off on those as well. With cereal, use oatmeal or barley or just skip it altogether. If she's on formula and not breastmilk (it's more common with formula because of the added iron, so forgive me if you're breastfeeding and I got that wrong) then you might want to consider switching to a gentle-ease type formula. I've had doctors talk about the good experiences they've had in just putting babies on that from day one. It's lower in lactose and more processed. It can help.
Sorbitol is the sugar in prunes that helps a baby to poop. It's not very digestible, so it basically moves right through you. The best foods to help with that are pears, prunes, peaches and pineapples (there's others of course, but those are easy to remember and have high amounts of sorbitol.) Pineapples aren't appropriate until they're a bit older, but the others you can definitely try. The juice can help, but you want to give it a little every day, not just once in a while or just when you notice she's constipated. But if you want variety then looking up sorbitol levels in foods can give you more options. You want just a little every day, to keep her regular. You can mix it with water if she doesn't like it straight, but with my daughter I just would give her 1.5-2oz a day mixed into her bottle, and that was more than enough (I started with 1/2 an oz and then increased slowly until I saw the amount she needed.) I'd probably do the slow introduction like I did, the whole 1/2oz, then increase by 1/2 every 2-3 days if she doesn't respond to that amount.
Personally, since she's 7 months old, I'd just buy some pureed pears/peaches/prunes, or buy some fresh fruit and mash it up at home and feed it to her that way. The extra fiber in the fruit will actually help move things along. Fiber is sort of like filler, the sorbitol makes it keep moving, so it's a pretty good natural combination and works a lot better than the juice to help with the hard poops. Keep in mind that unless the poop is dry or harder than peanutbutter, it's not real constipation.
The other thing to try is to get her on her feet, and get her off her butt. A lot of things out there put pressure on their rumps, and that can make pooping a bit harder. It's easier to poop standing up. She's 7 months old, and can't really stand up yet usually, but the more you can get her on her feet with you holding her by the hands or under the arms (not put in a bouncer where her rump where the support is, or a walker for the same reason) the better she'll move the poops through. When my daughter was having constipation issues I tried to make sure she was on her feet like that and bouncing or trying to walk for about 10 minutes of every hour at least (when she was awake.) Tummy time also definitely helps, as does swimming or being in the bath with you and helping her swim in there and kick. The more time off her butt she can have the more likely she'll be to poop, and the less hard they will be.
Add: Something else that you may want to consider is again if you are using formula, so forgive me if you're using breastmilk. Measuring mistakes are extremely common. Make sure that you fill the water while the bottle is on a level surface so that the water is level. Also make sure to take into account the meniscus, that's when the water makes a U or a bubble sort of on the top of something, it can throw off measurements. To minimize the meniscus, fill the bottle with water, get the water all along the walls of the bottle, and then empty and refill. You would be surprised how many parents short-change their baby maybe 1/8-1/10 of an oz of water every meal, and how that can add up over time and cause constipation.
Add: Just a note about juices. It really doesn't matter, so long as it's 100% juice to be honest. But you don't want 'less sugar' because it's that sorbital (a sugar) that you WANT them to have for the effects. So I wouldn't use 'motts for tots' in this instance. It just may not work as well. Gerbers infant juices do not add sugar that I'm aware of, it's all that natural sugar.