As far as ottos go, it's not at all unusual for them to be dropping like flies. Personally, I bought 9 of them wit the hopes of ultimately getting 4 or 5 to survive. From what I understand, they can appear to do perfectly well, and then die for no known reason within the first month or so. The ones that survive do great.
One problem with new ottos is that they are usually starved by the time you get them. Check their bellies. They should be rounded and plump. If they're flat or, even worse, caved in, then you need to work very hard to feed them. Forget the algae in the beginning. The first goal is to try to keep the ottos alive.
Also, I do my best to try to avoid any fish that has just been made available to be sold at the LFS. I want the left overs. The last of the bunch. I generally prefer that the fish have been up for sale for at least 10 days. That gives time for the diseased, weakened, and overly stressed fish to die out before I start making my selection. It also gives the fish more time to adjust between being caught (or bred), shipped and then stuffed in a crowded tank with little hiding places at the LFS in which people then gawk at them all day long. Rather stressful for a fish. And that's not even talking about all the water changes they experience in the process. So having a fish stay in a tank at the LFS for 10 or more days gives them a chance to recover before I stress them out all over again by having them caught, bagged, and then placed in yet another tank with different water, different fish, and different decor for the new fish to handle.
Okay, that's regarding the ottos. FEED them. Keep at it until you know they are fat and happy. Expect losses for at least the first month.
Second concern, your barbs. Unless fish become schizophrenic (they probably do), then odds are these guys are fighting. When fish fight, they don't have to cause any real harm to each other. It's like an argument. I've seen this with some of my fish. Sometimes my male guppies will get into an argument over who knows what. I've had a couple of cardinal tetras do it. I even had two brand new, less than a day old, guppy fry do it! It usually results in no damage to either fish, and they stop after a while. See if they're still doing it the next day. They may be trying to figure out which one is dominate. Once it'd determined, they each know their place in the group, and there's no need to continue arguing over it.
Last concern, cleaning your tank. Never do a "full cleaning" of your tank and filter at the same time. Always make sure that you leave a good amount of undisturbed bacteria to keep the water parameters in check. I'm very guilty of over cleaning. What I've found works best for me now is to (1) scrape the algae off the tank walls, (2) stop plunging in the vacuum hose deeply into the substrate, just get the top layer clean, and (3) do NOT change the filter until the following week (or so, depending on the type of filter). By doing things in this order, I feel that I am cleaning the tank most efficiently while still maintaining a healthy amount of bacteria. Cleaning the tank walls throws the crud into the water column. Which I then clean out when doing a water change and vacuuming. All the crud that is stirred up gets picked up by the filter which I then clean out the following week. It is the order that I think works best to get that super clean tank that some of us want without causing too much harm to the needed bacteria.
So to recap: Barbs? Ignore them. Ottos? Feed them. Tank cleaning? Do it in steps to keep bacteria colony plentiful.
Oh, I almost forgot. To keep the cucumber at the bottom, use plant weights. You know, those metal strips you get that hold a bunch of plants together? You can buy them, too. Just poke them through the cucumber in an upside down "U" shape so that the metal points are buried into the substrate. Others have used forks. I have some screwcumbers that work, too. Just make sure whatever you use it can be left in water for a day or two and is placed in such a way that it cannot scratch the fish as they eat further down into the cucumber.
I hope that helps!