Here's a comparison for you and some of the things I considered.
In my X10 set up, I had my aquacontroller and essential electronics plugged into a battery back up unit by APC. I had the APC battry backup unit plugged into a 15 amp noise filter and then into the wall. I needed to do this since the battery back up was plugged into the same outlet as X10. The battery backup unit has noise suppression circuitry which will soak up all X10 signals.
Next, I had a power strip (with no surge suppression features) plugged into the wall. Into the power strip, I plugged my computer line interface and all the control modules so they were all very close together electrically. The computer interface module was then plugged (with the phone cord) into the aquacontroller.
Further, I had to use X10 appliance modules so that they could handle inductive loads - sometimes the pumps I was using for wave makers would not work on the cheaper lamp modules.
I then had to modify the appliance modules to remove the "local control" feature. This feature allows you to turn on the appliance with the appliance on/off switch instead of sending an x10 signal. Unfortunately, it is prone to interference and with all the RF around a fish tank, it will turn the lights on and off by itself. I think this is probably the problem you are having. The Neptune website FAQ section has instructions how to remove this feature with pictures showing you the steps. So, I modified all my appliance modules this way.
Everything worked perfectly for quite some time. The only problem was the loud clicking noise every ten minutes when the wave pumps switched.
After awhile (six months or so) I started to experience some burned out x10 modules (they just quit working). Once you open one up to remove the local control feature you will see how cheaply made they are. I think they couldn't take the wave pump switching.
So, I switched. I went to Smarthome and bought their appliance modules. These modules are much better made and use better relays that have the added benefit of being about 80% quieter. But they are more expensive than the regular X10 modules.
I figured that I shouldn't go cheap with the X10 modules since they were controlling some major dollars in live stock etc. The bottom line is that you can't skimp on X10. My cost for the smarthome devices was $21.99 per appliance module (the silent kind), $29.99 for the 15 amp noise filter and about $19.99 for the computer line interface.
Now, incomparison to the DC8 (which has 8 plugs and is priced at $174.95), a set up with 8 Smarthome appliance modules as described above would run $225.90 all in ($21.99*8 + $29.99 (noise filter) + $19.99 (computer line interface)). You really need to compare these higher priced modules to the DC8 since the DC8 uses more expensive parts (like totally silent solid state relays - they are even quieter than the Smarthome modules).
Even showing a skewed comparison by using the cheaper X10 appliance modules you get close. I could not find the modules at $9.95. The X10 web site has the appliance modules at $12.99 or $13.99 depending on whether you need two pin or three pin. So using the $12.99 price you get $153.90 ($12.99*8 + $29.99 + $19.99). This is within $21.05 of the cost of the DC8 which is a much superior product and has NO interference or reliability issues like X10.
All in all, I think you can get X10 to work, but you can't succeed very well with it if you go cheap (unless you get lucky). And since the DC8 pricing gets you close, why risk messing something up with X10. Like I said above, it sounds like your only problem is the local control issue. Try fixing that and see if you have better luck with it. Hope this helps.
