Seriously you should not be covering up the whole pond with board without any holes. You should be using perforated board or board drilled with holes. The logic behind is when doing this the heat cannot escape under the hot sun light to the extend that your fish might be cooked. In science high temp water would loss heat through evaporation, by covering up the water will condense back and heat go back to the water. This is like a 'green house' effect or a heated tea pot with closed lid. No surprise you seeing condensation. Fortunately your fish still alive (I supposed your white background helps, if black gone case liao). I suggest the immediate action is to remove the board and replace with something with holes.

With regards to the nitrate is because the algae has been removed intentionally. Nitrate and algae growth goes hand in hand, striking a balance and checking one another. Removing the algae caused imbalanced and the nitrate to shoot up. Using the plant idea is good, like pandan or money plants. You can also consider getting those potted water plant (if you don't mind the soil). The only disadvantage of having plants is that the old leaf and roots will decay and rot, further causing ammonia and contamination issue and might even choke you filtration. Do remember to protect your pump by enclose it within a basket, as any of the leaves get in there will damage the pump.

Bro LauLanChu, really thanks for advises... Cannot imagine that I nearly kill all my fish by covering in this way 😓 also dont know why I forgot about all these science theory, maybe I am getting old 😅 I guess I have to remove the board and use back the netting (probably put double or triple layers) until I have a very good solution to this. Perforated board will obstruct the viewing from top?

Thanks those bros who know me and whatsapp me for the solutions... I appreciate very much. I am looking for more options where the covering will looks nice... Cheers.

I guess the thick glass or temperature glass with window frame....
Is a better option.
Risk if some one thro things down will shatter.

Can do like car bonnet actuation.... So can open or slant the glass

Regards to the heat.....when install leave a 1 inch gap higher so all rOund got
Vent Lor....or as maker to make holes

Ooo or sliding window style .... .... Else use those v thin Gauge kind 2cm by 2cm stainless steel mesh w sliding frame not so eye sore too....edge paint brown to match ur wood

maybe can try something like this?
instead of net, use acrylic or glass panel?

http://i284.photobucket.com/albums/l...4072010157.jpg

Hi Thomas, I have to say you have a very nice FGT setup.
However, you have to constantly overcome new challenges by means of experimenting new measures only to find out that it may solve one problem but creates other issues. I read this entire thread from the very beginning to the latest post before I went to bed last night. Honestly, I feel your joy and pain! :)
Now, if I may offer you a more practical approach to attempt to resolve these challenges, please consult a reputable landscaping contractor who has more experience in handling and maintaining ponds for condos and alike. It does no harm to listen or consider their suggestions before you make your next move.
You do have a very tricky situation given the surroundings, restrictions and limitations. We would be very interested to find out what clever solutions these professionals have to offer. Please consider this and keep us posted. Best of luck!

Hi Thomas, I borrow this pic from the goldfish folder. you may want to consider this type of cover for your tank. maybe use smaller holes opening for the netting.

http://farm9.staticflickr.com/8511/8...353225fd59.jpg
cheers.

yaa...pVC-surround..... or window frame style....

if u wana keep those acrylic sheets.... then consider drilling holes on them ;)

what kind of upstair neighbours u have...? all OK so far? no free-gifts from them?

I thought use pail to cover head.. :P

prevent TOPviewing :D:p