Changing Substrate

I suspect the water flow washed down the front of the tank and moved the sand that way. But the shape of the piled up sand doesn't quite make sense for that. So I wonder if it could have been the flow from the additional pump crashing into the flow washing the front of the tank.

I have a tank with a drilled bottom, so don't have any flexibility when it comes to moving the pipes. All I can do is adjust the height and angle of the outlet. I also have a small pump at the other end of the tank, so I've increased the height of that.

I'll have to keep an eye on things and expect that I'll have to make more tweaks before I get it right.

Thanks @fcmf . I do remember reading that link that you posted somewhere else (I can't remember where!). It is one of the things that got me seriously considering shrimp. I'm not interested in adjusting my water parameters, as I'm trying to minimise the maintenance on my tank. I'd rather chose stock that is happy in the water I have. But if I could get shrimp that would be happy in the water, then I'm happy to give them a food supplement.

Bearing in mind that I have rasboras that will eat all food that is out (even sinking pellets forge bottom feeders) (and I don't yet know how future fish purchases will behave), could you recommend a shrimp food? Or even a type? There seems so much choice, I wouldn't know where to start.

So I've discovered the cause of the substrate redistrution. Not the water flows, nor the kuhli loaches, but Bertie (my BN Plec).

I tidied up all the holes I could find, yesterday and it occurred to me that they are all near his favourite hangouts (not just in the sand, but also the finer gravel had been re-arranged.)

This afternoon I noticed that the sand under the newest cave had been shifted (it had been fine 10 minutes earlier!). Effectively enlarging the cave. And inside the cave, filling the space quite nicely was a BN!

I've just watched him in the hole at the front of the tank and he was swishing his large tail and I could seen sand flying about.

Hmm, I'll have to have a think about how to solve this problem. One of the caves I can make bigger, another I will just have to leave at the size Bertie makes it and smooth out the pile of gravel that he's moved out of the way.

I'm at a bit of a loss a to what do with the excavation at the front of the tank - other than keep filling it in. Or even just leave the divot and smooth out the excavated sand. I wasn't thinking of doing the latter until I just saw a kuhli feeding at the bottom of the hole - there must be something tasty down there!

At least I know the fish are enjoying the new set up. 

 
At least you have found the culprit.
That's the things with live animals, however much hard work you put in to creating a beautiful environment for them, they will still make their own minor adjustments.
My male BN also likes to rearrange areas around his main cave, and the gobies enjoy burrowing, but my mudskippers are the worst offenders. Their rearrangements in the tank are numerous, and have made it look more natural.
I think that Sue has a very valid point about the fish having a different concept of perfect when it comes to their environments. Unless it's going to cause a major problem in the tank, I'd be tempted to leave the rearrangements as they are, otherwise it may become a constant battle between you and Bertie.

Bertie leaning over to the other fish in the tank "look what she's done, just when I'd just got it perfect, she's gone and put it all back". 

I should add that all your comments about what Bertie is saying had me laughing out loud.