For as little of a pond as you are talking about, a single diffuser should be just fine, in fact it might be too much.

For the solar, you need to know how many kWh of energy you need each day. Then if you are going to use Lead acid batteries, multiply the amount you need by 2.5 times.

The 2.5 multiplier is to protect the batteries in winter. Never let your batteries get below 60% SOC in the winter. If they freeze, they are dead. Lead acid batteries will not freeze at 60% SOC until there are negative temps.

So let's say you are using a 50 watt pump 24/7. That is 1.2 kWh of energy used a day. You would like to protect your system for four days without sunlight. That is 4.8kWh. Now multiply that number by 2.5 and you get that you need a 12.5 kWh battery. 500 amp-hours is a lot of battery (12500 watt hours/24 volt battery rounded). Certainly, you can go with a smaller battery and protect for fewer days. You will also need a controller which can turn off the load to protect the batteries.

Does any of this make sense? If so, you are on your way.

Choosing the right charge controller is another matter. Let's get through step one first.

Brian

The one thing is the one thing
A dry fly catches no fish
Try not to be THAT 10%