If it doesn't appear to leak then I see no reason to seal it. Yes, you may as well, and should clean out most of the crud since it is easy to do while dry and empty. Like almost everything else on Gardenweb, it would be useful to know where you are and what garden zone you are in, both for plants and freezing issues.

A small or medium sized waterfall pump can be had for around $100 as I recall. However, you might want to spend more and get a mag drive pump that would use half the electricity. It doesn't appear that you have any plumbing running thru the cement from the lower to upper ponds so you would probably just get some flex tubing to connect a pump sitting in the lowest section back up to the upper pond going right over the wall into that upper area. You can hide the tubing with plants.

If you don't want fish, you can get mosquito dunks to prevent mosquitoes growing in your pond. I have a well and don't need dechlorination, but I suspect you could just let water sit for a few days before fish addition rather than using a dechlorinator; I suspect direct small topping off additions could be tolerated.

We are in South Carolina - our state motto is famously hot. I remember the pond would lose water due to evaporation in the summers. There are already frogs and salamanders living in the pond - another reason I was hesitant to mess with the muck but it's been pumped so that might have destroyed anything that was in there. There used to be a plum or some other flowering fruit plant by it that would provide some shade and we are thinking of replacing it (the plum succumbed to old age and fungus).

The pipes were connected and there used to be jets that would pull water up from the bottom and spit it back out but it's all fallen down into the pond and much of it is broken. I haven't figured out what the bottom pipe was for.

I guess that you could reset the spitter pipes but I don't really see how they would look all that nice. I had just assumed that you wished to have water spill from the upper area. I believe it would look and sound nice. You can put plants both in and around the pond. You can have an elephant ear plant in a big pot with the bottom slightly (or perhaps more) submerged. Potato vine will grow in a damp pot too. They both probably grow in the ground easy (and get huge) too; they will grow in North Carolina for me. Once you get some shade, impatiens will tolerate a pot in some water too. My EE's don't get growing until May, but perhaps they would grow sooner/faster for you and would be enough shade for impatiens.

Update**

Cleaned out the pond for the most part! There's still gunk in the waterfall pond and my pump won't get all the water out of the deep end (probably too much junk). I scooped junk and gunk all day but still wasn't about to move beyond this. Tomorrow I may just grab a bucket to dunk/dump water and finish up the deep end.

Long Pond · More Info

Four buckets of rocks and a grate later - this is what was in the upper waterfall pond. Does the brick and PVC contain chemicals harmful to fish and plants? If not I was just going to put everything back with a new grate. I think I'll replace the PVC pipe and find a pump to connect to it. The pipe connecting the waterfall pond and long pond is quite rusted but still flows. Is the rust harmful to fish or plants?

Long Pond · More Info

It is looking good. The remaining junk will settle to the bottom after you refill it and you will always have some vegetation that is decaying. Was the pump in the gravel with the grate over it? Was it setting at the lowest level of the pond? There are easier ways to make a filter for your pump and you don't want your pump to be on the very bottom if you have fish because if the water gets diverted out of the pond someway it will run the pond dry and the fish will die.

Plastic, gravel, bricks and rust won't hurt anything.

The pump was at the lowest level and would send water up to the bottom of the small waterfall pool. The water then moved up through the rocks, I guess as a way to filter it.

Thank you for the input, by the way!

I agree with Frankielynnsie on all points. I should have mentioned keeping a pump off the bottom to prevent accidental drainage. You could still put it in the lowest section but off the bottom, but it could go in the long section. The deep section might be useful for a water lily. They do better in deeper water. Or it could be used for a skimmer box if you didn't load up the rest of the pond with too many plants. Hand made or commercial skimmers are a container containing a screen basket that is connected to the pump input. The container is positioned just high enough within the pond so that a flat upper edge is just barely under water. The pump pulls a thin layer of water of the floating leaves and other crud into the skimmer where the leaves can be caught in the removable screen. OTOH, if a beautiful clean water surface is unimportant, the skimmer is unnecessary.

I would call a company that helped me .Koi smart / I think his name was John They have the pond filters system kits they can put together for your pond if that is what you need . Or give you the tools to make it work.